Fix Baseclass becomes! subclass.
[ci skip] fix "note" syntax
[ci skip] less derogatory explanation of defaults
[ci skip] add fixture docs link to guides

The fixtures API docs are really comprehensive. We should either be driving people there, or perhaps making fixtures its own guide.
[ci skip] explain association behavior. 

When you manually specify `id:` attribute in a fixture Rails' built in association assignment feature will no longer work. http://stackoverflow.com/questions/23985675/incorrect-association-in-fixtures

This commit raises awareness of this behavior and encourages developers to look directly at the fixture documentation.
Bring type casting behavior of hstore/json in line with serialized

`@raw_attributes` should not contain the type-cast, mutable version of
the value.
[ci skip] remove double "in your"
Removed CaptureHelper#flush_output_buffer as it is only used in tests.
reviewed: @tenderlove
Fix behavior of handling BC era dates.

BC era year is (astronomical year + 1) and starts from 1 BC.
document include_blank's usage with a string [ci skip]
Mime::PNG is already defined.
Merge pull request #15521 from edogawaconan/fix_bc_postgres_master

Fix BC year handling in postgres
Merge pull request #15529 from zuhao/refactor_actionpack_mime_type_test

Make sure Mime::Type is not altered after tests.
Reset callbacks after test.

Otherwise the state of callback chain is leaked.
Avoid hard-coded value in test setup/teardown.
Restore Response.default_headers after test.
Merge pull request #15531 from zuhao/refactor_actionpack_request_test

Avoid hard-coded value in test setup/teardown.
Merge pull request #15532 from zuhao/refactor_actionpack_response_test

Restore Response.default_headers after test.
Merge pull request #15523 from gregmolnar/doc_fix

document include_blank's usage with a string [ci skip]
Merge pull request #15513 from zenspider/remove_flush_output_buffer

Removed CaptureHelper#flush_output_buffer as it is only used in tests.