Fixture named accessors do not work when the fixture name is not the table name

If you have a fixture name which does not match the table name for the associated model, the fixture named accessors will raise an exception like this:

NoMethodError: undefined method `[]' for nil:NilClass

The underlying cause is similar to ticket #8 Fixture caching does not work with prefixed/suffixed tables: @loaded_fixtures has values stored in it using the table name, but loaded using the fixture name. The attached patch (with test) fixes this.

(The test case is simpler than a real-life situation, but illustrates the problem. A real-life situation might involve prefixed/suffixed tables or tables in different schemas.)