Fix PasswordField on postgres (fix #766)
@coleifer any feedback on this? :)
 LGTM. I haven't tested with Postgres, but the changes looks sane. Changing from BaseModel to TestModel allows it to be ran with `--engine postgres`, and changing from TextField to BlobField allows bytes to be stored safely. Needs sign off from @coleifer 
 Thank you!

Fix #766

This also makes the PasswordField tests to be ran with postgres.
