Make the default form method GET

The default method for forms is GET, not POST [1][2].  This changes
kerodon's behaviour to GET rather than POST when a form element doesn't
specify a method attribute.

[1]: http://www.w3.org/TR/html401/interact/forms.html#h-17.3
[2]: http://www.w3.org/TR/html5/forms.html#attr-fs-method