Improve docs, changelog and release notes for Action Mailer default_options=

[ci skip]
Fix failing AM test due to missing template
Remove unused code.
Added *instance_writer: false* for stored/serialized attributes.
Merge pull request #6987 from dmathieu/limit_inspect

Limit the number of records in Relation#inspect
Update master
Merge pull request #6997 from kennyj/fix_stored_attributes

Added *instance_writer: false* to stored/serialized attributes.
Merge pull request #6996 from kennyj/remove_unused_code_20120707

Remove unused code.
Merge pull request #6993 from morgoth/engine-table-name-prefix-generator-fix

Engine table name prefix generator fix
Added support add_flash_types
Merge pull request #6992 from kennyj/improve_6977-2

Allow people to register their own flash types.
Simplify Relation#inspect
Relation#inspect handles doesn't perform a new query on an already-loaded relation
fixup changelog
Use "instance_accessor" for flash types class attribute
Merge pull request #6838 from kennyj/added_registration_task

Allow to register database tasks from different adapters
Merge pull request #6938 from acapilleri/as_json_refactoring

Small refactoring of as_json method
Load all records in Relation#inspect

A test was failing due to the way that Relation#inspect causes
association proxies to ignore unsaved records added to the association.

This is fixed by simply calling to_a and letting to_a figure out how to
get the records (which, in the case of associations, takes into account
new records).

I think it is acceptable to do this rather than limiting the query at
the database level:

* It's what we've done in all released Rails versions up to this point
* The goal of the limit is to not flood the console with output - this
  is the problem we're targeting, rather than the actual loading of the
  records from the database
* You probably want to do something with those records later anyway,
  otherwise you wouldn't have built a relation for them.
Refactor locked? method in query cache

Introduced in 75b340d1a4bcf2f1233fb65a15ff6b8059e2230e
Revert "Add nodoc to relation methods"

This reverts commit c47a698d5d497340d4e349257522212173865838.

Reason: Let's revert pending further discussions