Submit button fixes

[thet]

Fixes a problem recently reported by @cornae

• Submitting pat-inject enabled forms without an action attribute but a submit button with a formaction attribute would break pat-ajax.

This fixes also two other problems I found on the way:

• Submitting with a formaction-button called a pat-ajax method twice.
• pat-inject did not support submit buttons without an explicit type="submit" attribute, e.g. <button>, which is implicitly a submit button.