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.
Merge pull request #12009 from wangjohn/fixtures_fix

Removing instances of string class_names in fixtures.