Fix primary key behaviour in bulk_updatepr/181

Suppose you have a model class with a primary key.

Base = declarative_base()
class User(Base):
  id = Column(BigInteger, primary_key=True)
  name = Column(String)

Previously, running
`bulk_update_mappings(User, {'id': 1, 'name': 'hello'})`
would emit the following:

```UPDATE users SET id=1, name='hello' WHERE id=1```

This is contrary to the stated behaviour, where primary keys are omitted
from the SET clause. Furthermore, this behaviour is harmful, as it
can cause the db engine to lock over-aggresively (at least in Postgres).

With this change, the emitted SQL is:

```UPDATE users SET name='hello' WHERE id=1```

1 files changed, 34 insertions, 4 deletions

diff --git a/test/orm/test_bulk.py b/test/orm/test_bulk.py
index e27d3b73c..1e0a735c7 100644
--- a/test/orm/test_bulk.py
+++ b/test/orm/test_bulk.py
@@ -96,11 +96,41 @@ class BulkInsertUpdateTest(BulkTest, _fixtures.FixtureTest):
 
         asserter.assert_(
             CompiledSQL(
-                "UPDATE users SET id=:id, name=:name WHERE "
+                "UPDATE users SET name=:name WHERE "
                 "users.id = :users_id",
-                [{'users_id': 1, 'id': 1, 'name': 'u1new'},
-                 {'users_id': 2, 'id': 2, 'name': 'u2'},
-                 {'users_id': 3, 'id': 3, 'name': 'u3new'}]
+                [{'users_id': 1, 'name': 'u1new'},
+                 {'users_id': 2, 'name': 'u2'},
+                 {'users_id': 3, 'name': 'u3new'}]
+            )
+        )
+
+    def test_bulk_update(self):
+        User, = self.classes("User",)
+
+        s = Session(expire_on_commit=False)
+        objects = [
+            User(name="u1"),
+            User(name="u2"),
+            User(name="u3")
+        ]
+        s.add_all(objects)
+        s.commit()
+
+        s = Session()
+        with self.sql_execution_asserter() as asserter:
+            s.bulk_update_mappings(
+                User,
+                [{'id': 1, 'name': 'u1new'},
+                 {'id': 2, 'name': 'u2'},
+                 {'id': 3, 'name': 'u3new'}]
+            )
+
+        asserter.assert_(
+            CompiledSQL(
+                "UPDATE users SET name=:name WHERE users.id = :users_id",
+                [{'users_id': 1, 'name': 'u1new'},
+                 {'users_id': 2, 'name': 'u2'},
+                 {'users_id': 3, 'name': 'u3new'}]
             )
         )