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
document include_blank's usage with a string [ci skip]
Reset callbacks after test.

Otherwise the state of callback chain is leaked.
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.
Merge pull request #15530 from zuhao/refactor_actionpack_reloader_test

Reset callbacks after test.
Avoid misuse of underscore argument

Per convention, underscore-only argument names should be used for unused parameters.
Merge pull request #15535 from coreyward/patch-1

Avoid misuse of underscore argument
Remove optimization that was required with whiny nils

Whiny nils is no longer a thing, so we no longer need this optimization
Merge pull request #15536 from sgrif/sg-remove-whiny-nil-thing

Remove optimization that was required with whiny nils
Use a frozen Set instance for CGI_VARIABLES.

Also expand the CGI_VARIABLE name listing to multiple lines for cleaner diffs and legibility.
Merge pull request #14971 from versioncontrol/#14785

Baseclass becomes! subclass
docs, update `includes` + `where` example in AR guide. [ci skip]

Closes #15541.