updated FormBuilder class to prevent passwords from being filled from flash

Passwords were filling from flash data when redirecting using withInput()

I believe that this is a security issue since all it takes is for anyone to use withInput() anywhere that a password is referenced. 

The password isn't being set from Form::model(). It's being done automatically. Many inexperienced developers would simply not even know where it's coming from or assume that it's their browser cache.