Fixed #4492 - Provide tests for mixed-case column names

https://code.djangoproject.com/ticket/4492
I created a model for testing mixed case entity names + a suite of 4 tests that test that. Since PostgreSQL lowercases everything not in quotes, usually only the last test would fail if this isn't implemented properly in the drivers.
Tested the unit-tests on PostgreSQL 9.2 and SQLite