Cannot validate raw integer 0 with required IntegerField.#39

I'm having issues validating a required IntegerField when the JSON value passed is 0: ``` python import wtforms_json wtforms_json.init() from wtforms import Form, IntegerField from wtforms.validators import InputRequired class MyForm(Form): a = IntegerField(validators=[InputRequired()]) # Works with > 0 form = MyForm.from_json({'a': 5}) assert form.validate() # Does not work with value 0 form = MyForm.from_json({'a': 0}) assert form.validate() # Works with string '0' form = MyForm.from_json({'a': '0'}) assert form.validate() ``` Using wtforms 2.0.2