Removed a confusing duplicate SESSION_COOKIE_DOMAIN header

The note is clearly a part of MESSAGE_STORAGE documentation. As a separate
section, it broke automatic link generation on the HTML version of the
documentation.
Fixed #20472: response.content should be bytes on both Python 2 and 3
Fixed #20212 - __reduce__ should only be defined for Py3+.
Merge pull request #1191 from ambv/content_is_bytes

Fixed #20472: response.content should be bytes on both Python 2 and 3
Merge pull request #1190 from ambv/doc_settings_fix

Removed a confusing duplicate SESSION_COOKIE_DOMAIN header
Use assertIsInstance in tests.

Gives much nicer errors when it fails.
Merge pull request #1192 from mjtamlyn/assertIsInstance

Use assertIsInstance in test code.
Fixed test failures from 09f865276554f35060ff939722ec4cefd578edf6.
Fixed a Python 2.6 regression (GzipFile can't act as a context manager)
Fixed #17308 -- Enabled the use of short_description on properties in the admin.
Merge pull request #1194 from ambv/python26_is_fun

Fixed a Python 2.6 regression (GzipFile can't act as a context manager)
Imported copyreg from six.moves
Fixed #6412 -- More details if a template file cannot be loaded

Report more details about template files in loader postmortem.