[html-formfu] Always HTTP Method POST

[Rod Taylor]

I want to ensure the form is submitted with the HTTP Method of POST
(or potentially PUT or DELETE) but never GET.

I do not see an easy way of adding a Constraint for this type of
thing. method() sets the method when creating the form but does not
appear to enforce it to be correct for submitted_and_valid() to be
set.

A single flag I can set in

Thanks,

Rod