Test multiple submit buttons submit correct value.

Currently throwing InvalidArgumentException: Unreachable Field 'button2'
Fix issue InvalidArgumentException if more than one submit with different names on form.
Remove unnecessary use that snuck in due to autocomplete.
Additional test (failing) to test issue with wrong submit button being sent after fillField() has been used.
Fixes #1051 - Form with multiple submit buttons will submit correct values.

We do not know which submit button was pressed until late on. getFormFor() is too early as that is used to retrieve the Form for lots of elements.
Add the submit button value in submitFormWithButton().
Merge pull request #1369 from ixisandyr/unreachable_field_bug

Test multiple submit buttons submit correct value. refs #1368