Form::sign() shouldn't include 'label' fields. (mentioned in issue #998 part 1)
Merge pull request #1132 from koushki/bug/LabelInFormSign

Fixing FormSignature::check() when 'label' is added with form helper
Making form signature tests not depend on string algo.
Adding more FormSignature tests. Proves 2. of #998.
Fixing test where FormSignature would fail on fields.

Fixes 2. of issue #998.

Refactoring into symmetric compile/parse methods.