test case for default current_timestamp

@koenpunt 
@EVILoptimist

This illustrates the following MYSQL Adapter bug:

1. Create a model with a column of type "timestamp" (or datetime), and default of "CURRENT_TIMESTAMP"
2. Instantiate a new model, and try to access the column

Expected result: 
You get back a DateTime, set to the current time

Actual result:
You get back null