b71039660b made FormField.setType(Type)
throw an illegal argument exception if type is fixed, but the
DataFormProvider was not changed, so it still would call
setType(Type.fixed).
Change DataFormProvider so that the non-argument constructer of
FormField is used when type == fixed.
Data Forms Validation are a part of Data Fields and implemented as
extensions, added to a Datafield.
Data validation extensions are validated before adding to the message,
using the consistency rules as described in the XEP.
Fixes SMACK-621.
Minor modifications done by Florian Schmaus <flo@geekplace.eu>
