Make MySQL custom port work

Hi,

I took a look into the code and found that all connection fields are forced into String. This does not work with the Active Record MySQL Adapter, since it requires Integer for the port number. I tried to fix it by introducing a `STRING_FIELDS` variable that includes all fields that should be forced into Strings (currently only `adapter`). I added a test for the port number, all other tests are still passing.

I know it is not the prettiest solution, so suggestions for improvement are very welcome.

Thanks!