Capturing the function, and returning a new object from a class that is
defined within the capturing function seems to cause some sort of really
pathological behavior that causes lazify() to take up a bunch of CPU
time, especially when it is part of a hot code path.

Thanks to Martijn Faassen for reporting!

Closes #282

Refactored the logic in Response.__init__ to handle default charset more
consistently.

Added logic in the charset setter that ignores attempts to set it on
JSON content types.

Removed explicit charset specification from exceptions since this is
handled correctly for the text types within Response.

Fixed some affected tests, and added assertions for content types in
exceptions.

Addresses https://github.com/Pylons/webob/issues/237

self.body has the side-effect of attempting to read the entire iterable
(if it exists), which is not a very nice thing to do. has_body will
instead simply return True or false without that nasty side effect.

Currently if WebOb encounters a */* Accept header on an exception it
will return an application/json result. This leads to some interesting
results for crawlers and others that don't send an explicit HTTP Accept
and then end up receiving JSON instead of what was previously HTML.

This breaks B/W compat with issue #230.

Accept does not take into account type/subtype, and thus could
potentially return the wrong result to a client if they send a funky
Accept header.

Closes #240

Add more tests to bring coverage back up to 100%

Closes #209