Does not work with flask-admin due to override of formdata#24

The specific issue is that if wtforms_json is used, the [monkey patch](https://github.com/kvesteri/wtforms-json/blob/master/wtforms_json/__init__.py#L179) does not allow fallthrough/bubble-up of different datatypes. For example, if using flask-admin and a StringField is overridden on the form as a FileUploadField, the formdata processor will normally parse the provided data, save the file, and store the filename to the StringField. The monkey patch breaks this process since it transforms all input types into their text representation via `six`: ``` if isinstance(self, TextField): if self.data is None: self.data = u'' else: self.data = six.text_type(self.data) ``` This is obviously incorrect. I'm not sure about the motivations for this code, so can't say what the correct solution would be. I actually suspect that this block of code should be removed altogether, since it's certainly possible that the back-end should store no value if that's what's received - rather than a blank string. Or it should be handled as the model expects, rather than as overridden text. I'm happy to submit a pull request, but wanted to first understand what the intentions were.