remove #to_s on values with nil defaults

When you don't specify values for fields of type time or date, they blow up with an error message similar to:

```
-[String isEqualToDate:]: unrecognized selector sent to instance 0x171e7f10
```

This is because the default value from the gem was always being turned to `String` even when no default value was provided.

Please note, I was not actually able to run the tests locally. With the latest upgrade of RubyMotion and the version lock of the gem, something is happening, which I did not want to take the time to evaluate.