Fix app generator test hanging.
Merge pull request #15150 from xtian/fix-railties-tests

Fix app generator test hanging.
Add missing tests for column type cast behavior
Add a type object to Column constructor

Part of #15134. In order to perform typecasting polymorphically, we need
to add another argument to the constructor. The order was chosen to
match the `oid_type` on `PostgreSQLColumn`.