Bring back some bits of documentation for scopes which were removed as part of the reversion in 256b363e

Revert "Deprecate defining scopes with a callable (lambda, proc, etc) via the scope class method. Just define a class method yourself instead."

This reverts commit f0e198bf.

Conflicts:

	activerecord/test/models/post.rb

activesupport/test/json/encoding_test.rb need Json to pass against with Jruby

* 'identity_map' of https://github.com/jasoncodes/rails:
  Bypass IdentityMap in PostgreSQL geometric tests.

The identity map cache prevents us from seeing the DB formatted strings.

Add more tests, ensure we never yield outside the fiber context and that we swap buffers when moving from parent to child.

Yo dawg, I heard you like streaming. So I put a fiber, inside a block, inside a body, so you can stream.

Return nil from read_attribute(:foo) if 'foo' is not present in the @attributes hash, but the _foo method has been defined. This brings the behaviour into line with the 3-0-stable branch and the master branch before 93641ed6 (there were previously no assertions about this which is why the change slipped through). Note that actually calling the 'foo' method will still raise an error if the attribute is not present.

Undo performances regressions I introduced in bbe0a507 and add test for an edge case. Add comments to explain the intent of the code.