Add documentation for ActiveRecord::Observer.

The instructions about configuration covered only rails-specific usage.

add second t to overwritten

fixing a misspelling in the documentation

Fix code comment to reflect its intent.

Add teardown method to AR::Mig::RenameTableTest

Dry up reseting the renamed table after each test.

Also made use of the AR::Base.connection object already
available from AR::MigrationTest#connection.

When someone sends malformed authorization header, like:

    Authorization: Token foobar

given token should be just ignored and resource should not be authorized,
instead of raising error. Before this patch controller would return 401 header
only for well formed tokens, like:

    Authorization: Token token=foobar

and would return 500 in former case.

Fixed bug creating invalid HTML in select options

Rename default sequence when table is renamed? [AR:postgres]

When a select tag is created for a field with errors, then the inserted
options will errantly have a <div class="field_with_errors"> wrapping
them.

AM::Validations: remove documentation duplicates

minitest/spec provides `describe`, so deprecate the rails version and
have people use the superclass version

The extension was removed in 22bc12ec,
making "test" an alias for minitest's "it".

[Guide] Fix broken links

Add references statements to migration generator

Add a paragraph about references statements in migration generator
and the subchapter "Supported type modifiers"

AddXXXToYYY/RemoveXXXFromYYY migrations are produced with references
statements, for instance

    rails g migration AddReferencesToProducts user:references
supplier:references{polymorphic}

will generate the migration with:

    add_reference :products, :user, index: true
    add_reference :products, :supplier, polymorphic: true, index: true

Remove rails implementation of describe, alias "test" to "it"

Fix Bleak House link in debugging guide. Closes #7002

Follow the consistency defined in dbc43bcc.

Changes introduced in 7404cda9.
Fix railties build.

Add few information on Fedora and CentOS [ci skip]

Show Routes while Debugging RoutingError