Revert "Revert "Merge pull request #16059 from jenncoop/json-serialized-attr""

This reverts commit 6f3c64eeb1dc8288dae49f114aaf619adc7dcb7f.

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #16162 from chancancode/fix_json_coder

Fixed JSON coder when loading NULL from DB
Fixed SQL syntax for postgresql
Document the change in `nil` handling for serialized attributes

Also updated the test case to reflect that
Merge branch 'master' of git://github.com/rails/rails

Conflicts:
	activerecord/CHANGELOG.md