Override Delegator#respond_to_missing? to work with private methods

The database_cleaner gem uses respond_to? to check if the connection responds to `rollback_transaction_records` which is a private method, which generates a warning each time the clear method is called (which is usually on every test), and also send to call the private method.

This PR overrides Delegator#respond_to_missing? and adds #send and #private_methods to work with private methods in the connection proxy.

/cc @osheroff @grosser @pschambacher @bquorning @adammw