Don't to invoke default form helper on typeless elements

If the type of the form renderer cannot be optained, the render method
of the form element helper reached for the 'form_' helper and passed an
element to it that caused an exception. Instead, the parent render
method should be called.
Merge pull request #4 from fidelski/master

Don't to invoke default form helper on typeless elements (e.g. Captcha)