title: Flask-WTForms throws error for IntegerField instead of failing validation
body: <p>When I create a form using wtf_forms and Flask-WTF and use the IntegerField input, I can't use it in combination with the Length validator </p>
  
  <p>If I remove the Length restriction then it works fine. Surely I should be able to apply a Length validation to an IntegerField?</p>
  
  <p>Python Code. </p>
  
  <pre><code>from flask_wtf import Form
  from wtforms import TextField, PasswordField, IntegerField, validators
  
  class RegistrationForm(Form):
      firstname = TextField('First Name', [validators.Required()])
      lastname = TextField('Last Name', [validators.Required()])
      telephone = IntegerField('Telephone', [validators.Length(min=10, max=10, message="Telephone should be 10 digits (no spaces)")])
  
  TypeError
  TypeError: object of type 'int' has no len()
  
  Traceback (most recent call last)
  File "C:\Python27\lib\site-packages\flask\app.py", line 1701, in __call__
  return self.wsgi_app(environ, start_response)
  File "C:\Python27\lib\site-packages\flask\app.py", line 1689, in wsgi_app
  response = self.make_response(self.handle_exception(e))
  File "C:\Python27\lib\site-packages\flask\app.py", line 1687, in wsgi_app
  response = self.full_dispatch_request()
  File "C:\Python27\lib\site-packages\flask\app.py", line 1360, in full_dispatch_request
  rv = self.handle_user_exception(e)
  File "C:\Python27\lib\site-packages\flask\app.py", line 1358, in full_dispatch_request
  rv = self.dispatch_request()
  File "C:\Python27\lib\site-packages\flask\app.py", line 1344, in dispatch_request
  return self.view_functions[rule.endpoint](**req.view_args)
  File "C:\index.py", line 45, in submit
  if form.validate_on_submit():
  File "C:\Python27\lib\site-packages\flask_wtf\form.py", line 156, in validate_on_submit
  return self.is_submitted() and self.validate()
  File "C:\Python27\lib\site-packages\wtforms\form.py", line 271, in validate
  return super(Form, self).validate(extra)
  File "C:\Python27\lib\site-packages\wtforms\form.py", line 130, in validate
  if not field.validate(self, extra):
  File "C:\Python27\lib\site-packages\wtforms\fields\core.py", line 175, in validate
  stop_validation = self._run_validation_chain(form, chain)
  File "C:\Python27\lib\site-packages\wtforms\fields\core.py", line 195, in _run_validation_chain
  validator(form, self)
  File "C:\Python27\lib\site-packages\wtforms\validators.py", line 91, in __call__
  l = field.data and len(field.data) or 0
  TypeError: object of type 'long' has no len()
  </code></pre>