Invalid keys shouldn't cause a crash

Keys of the form name-notdigits will cause the validation to crash.

In theory this is a security issue because an attacker could submit an
arbitrary form with keys in this format.
Merge branch 'master' of https://github.com/f0rk/formencode into f0rk-master
Variable decode tests and fix isdigit check.
Merge branch 'master' of github.com:formencode/formencode