Document nested attributes as hash of hashes

Document the possibility to use a hash of hashes for nested attributes for a one-to-many association (in addition to the documented possibility to use an array of hashes).

Align indentation in comments.
Merge pull request #9688 from alexeymuranov/doc-nested-attributes-3-2

3-2-stable documentation: the possibility to use a hash of hashes for nested attributes
Fix a fixtures test case with table prefix/suffix

Make sure the table name of a model is reset in a test case after assigning ActiveRecord::Base.table_name_prefix and ActiveRecord::Base.table_name_suffix.
Use the correct table name from fixture

Fixed a bug in fixtures.rb where the table name was incorrectly inferred from the fixture path which was ambiguously called "table_name" but was also used as the fixture name.
Now, the "correct" table name is taken from an instance variable.
Test fixtures with custom model and table names

Test using fixtures with random names and model names, that is not following naming conventions but using set_fixture_class instead.

It is expected that the table name be defined in the model, but this is not explicitly tested here.  This will need to be fixed.
"foo/bar" instead of "foo_bar" keys for fixtures

This solves an issue with set_fixture_class class method caused by create_fixtures method's overwriting passed to it fixture model classes, when the fixtures are "namespased": foo/bar or admin/users.

The idea is to use "foo/bar" string as the name and identifier of a
fixture file bar in directory foo.  The model class of the fixture is either set with set_fixture_class method, or otherwise inferred from its name using camelize method.

Also a bug is fixed in lines 487-489 when the table names were guessed by substitution from the fixture file names, ambiguously called  table_names, instead of being taken from fixture attributes.  Now they are taken from attributes.

Add a method `ActiveRecord::Fixtures.default_fixture_model_name` instead of a ":nodoc:" class method `ActiveRecord::Fixtures.find_table_name`, which does not correspond to its name.  The old method name is aliased to the new one, and is deprecated in master in favour of the new one.
Test case: fixture table name is defined in model
Fixture's table name be defined in the model

Made the fixture's table name be taken from its model class whenever this class responds to table_name, instead of inferring it sometimes from the fixture's pass.

The previous behavior seemed buggy because it depended on whether the model class was passed as a constant or as a name string.

Improved Fixtures#initialize.
Fixes for TestFixtures::setup_fixture_accessors

Renamed "fixture_name" to "accessor_name" and made fixture names in the form "admin/users" be used as the key for the hashes @fixture_cache and @loaded_fixtures.

Previously the variable "fixture_name" here was getting a value of the form "admin_user", and this value was then used as the hash key.