Removing instances of string class_names in fixtures.

Also, constantizing the default_fixture_model_name when it gets loaded
in from the file. Later, when the class_name is passed to a new
FixtureSet, a deprecation warning will occur if the class_name is a
string.
Changing deprecation_horizon to be Rails 4.2

Also, +ActiveRecord::Migrator.proper_table_name+ should actually have a
deprecation horizon of Rails 4.2 (not 4.1).
Merge pull request #12009 from wangjohn/fixtures_fix

Removing instances of string class_names in fixtures.
Merge pull request #12014 from wangjohn/change_deprecation_horizon

Changing deprecation_horizon to be Rails 4.2
check class hierarchy with is_a? in PredicateBuilder.expand