createField: Allow to control a fallback value of initialValue

[kettanaito]

What

I suggest to add an option to createField which would control a fallback value of initialValue (which is registeredValue internally).

Why

The fallback value of registeredValue is a hard-coded empty string at the moment. That causes issues with fields which expect a different data type, rather than string (i.e. datepickers).

How

1. Add a new option.
2. Provide it instead of an empty string.
3. Write respective test(s).

[kettanaito]

Merged.

Ready to be released in the next minor release version. Closing.