Serialize form data if we are passed a class that can be serialized

[rossriley]

Fixes #248

This is mainly to handle image uploads since they get to the Contenttype handler as an instance of Submission\File rather than an array.

At this point the field save will not work so we can call the jsonSerialize method to get the prepared data. Using this approach allows other custom field handlers in the future to do the same if needed.

[bobdenotter]

👍