Rewrite CaseInsensitiveDict to work correctly/sanely

Fixes #649 and #1329 by making Session.headers a CaseInsensitiveDict,
and fixing the implementation of CID. Credit for the brilliant idea
to map `lowercased_key -> (cased_key, mapped_value)` goes to
@gazpachoking, thanks a bunch.

Changes from original implementation of CaseInsensitiveDict:

1.  CID is rewritten as a subclass of `collections.MutableMapping`.
2.  CID remembers the case of the last-set key, but `__setitem__`
    and `__delitem__` will handle keys without respect to case.
3.  CID returns the key case as remembered for the `keys`, `items`,
    and `__iter__` methods.
4.  Query operations (`__getitem__` and `__contains__`) are done in
    a case-insensitive manner: `cid['foo']` and `cid['FOO']` will
    return the same value.
5.  The constructor as well as `update` and `__eq__` have undefined
    behavior when given multiple keys that have the same `lower()`.
6.  The new method `lower_items` is like `iteritems`, but keys are
    all lowercased.
7.  CID raises `KeyError` for `__getitem__` as normal dicts do. The
    old implementation returned
6.  The `__repr__` now makes it obvious that it's not a normal dict.

See PR #1333 for the discussions that lead up to this implementation

1 files changed, 63 insertions, 26 deletions

diff --git a/requests/structures.py b/requests/structures.py
index 05f5ac15..8d02ea67 100644
--- a/requests/structures.py
+++ b/requests/structures.py
@@ -9,6 +9,7 @@ Data structures that power Requests.
 """
 
 import os
+import collections
 from itertools import islice
 
 
@@ -33,43 +34,79 @@ class IteratorProxy(object):
         return "".join(islice(self.i, None, n))
 
 
-class CaseInsensitiveDict(dict):
-    """Case-insensitive Dictionary
+class CaseInsensitiveDict(collections.MutableMapping):
+    """
+    A case-insensitive ``dict``-like object.
+
+    Implements all methods and operations of
+    ``collections.MutableMapping`` as well as dict's ``copy``. Also
+    provides ``lower_items``.
+
+    All keys are expected to be strings. The structure remembers the
+    case of the last key to be set, and ``iter(instance)``,
+    ``keys()``, ``items()``, ``iterkeys()``, and ``iteritems()``
+    will contain case-sensitive keys. However, querying and contains
+    testing is case insensitive:
+
+        cid = CaseInsensitiveDict()
+        cid['Accept'] = 'application/json'
+        cid['aCCEPT'] == 'application/json'  # True
+        list(cid) == ['Accept']  # True
 
     For example, ``headers['content-encoding']`` will return the
-    value of a ``'Content-Encoding'`` response header."""
+    value of a ``'Content-Encoding'`` response header, regardless
+    of how the header name was originally stored.
 
-    @property
-    def lower_keys(self):
-        if not hasattr(self, '_lower_keys') or not self._lower_keys:
-            self._lower_keys = dict((k.lower(), k) for k in list(self.keys()))
-        return self._lower_keys
+    If the constructor, ``.update``, or equality comparison
+    operations are given keys that have equal ``.lower()``s, the
+    behavior is undefined.
 
-    def _clear_lower_keys(self):
-        if hasattr(self, '_lower_keys'):
-            self._lower_keys.clear()
+    """
+    def __init__(self, data=None, **kwargs):
+        self._store = dict()
+        if data is None:
+            data = {}
+        self.update(data, **kwargs)
 
     def __setitem__(self, key, value):
-        dict.__setitem__(self, key, value)
-        self._clear_lower_keys()
+        # Use the lowercased key for lookups, but store the actual
+        # key alongside the value.
+        self._store[key.lower()] = (key, value)
 
-    def __delitem__(self, key):
-        dict.__delitem__(self, self.lower_keys.get(key.lower(), key))
-        self._lower_keys.clear()
+    def __getitem__(self, key):
+        return self._store[key.lower()][1]
 
-    def __contains__(self, key):
-        return key.lower() in self.lower_keys
+    def __delitem__(self, key):
+        del self._store[key.lower()]
 
-    def __getitem__(self, key):
-        # We allow fall-through here, so values default to None
-        if key in self:
-            return dict.__getitem__(self, self.lower_keys[key.lower()])
+    def __iter__(self):
+        return (casedkey for casedkey, mappedvalue in self._store.values())
 
-    def get(self, key, default=None):
-        if key in self:
-            return self[key]
+    def __len__(self):
+        return len(self._store)
+
+    def lower_items(self):
+        """Like iteritems(), but with all lowercase keys."""
+        return (
+            (lowerkey, keyval[1])
+            for (lowerkey, keyval)
+            in self._store.items()
+        )
+
+    def __eq__(self, other):
+        if isinstance(other, collections.Mapping):
+            other = CaseInsensitiveDict(other)
         else:
-            return default
+            return NotImplemented
+        # Compare insensitively
+        return dict(self.lower_items()) == dict(other.lower_items())
+
+    # Copy is required
+    def copy(self):
+         return CaseInsensitiveDict(self._store.values())
+
+    def __repr__(self):
+        return '%s(%r)' % (self.__class__.__name__, dict(self.items()))
 
 
 class LookupDict(dict):