Use upper case to represent HTTP methods for forms

Per [RFC 2616 section 5.1.1][ietf], HTTP methods are case-sensitive.

- Change the internal representation of the form's method to upper case
- Retain lower case for Form#FormMethod(), because that is intended for
  X/HTML
- Update FormTest to accommodate the case changes

[ietf]: http://tools.ietf.org/html/rfc2616#section-5.1.1