title: WTForms SelectMultipleField: disable validation
body: <p>I have a  SelectMultipleField that is populated/modified at runtime using JavaScript. My problem is that this violates the validator, hence the call to <code>form.validate_on_submit()</code> always fails.</p>
  
  <pre><code>SelectMultipleField('Test', coerce=int)
  </code></pre>
  
  <p>I already tried to set my custom validator that does nothing:</p>
  
  <pre><code>def dummy_validator(form, field):
      print("check: "+str(field.data))
  
  SelectMultipleField('Test', coerce=int, [dummy_validator])
  </code></pre>
  
  <p>However this still fails. For example I get the error message <code>'7' is not a valid choice for this field</code>.</p>
  
  <p>What is needed to make SelectMultipleField pass the validation process started by <code>form.validate_on_submit()</code>?</p>