Added a test for submitting forms without a submit button

Refs https://github.com/Behat/Mink/issues/497

All drivers except MinkBrowserKitDriver are currently able to use ``submitForm`` on a form without submit button. And while trying to make a test reaching the exception case in MinkBrowserKitDriver, I discovered that DomCrawler (and even our driver) is able to submit a form without submit button.
The restriction is only in our driver code, which prevents manipulating fields if there is no submit button: we can submit the form, but we cannot change the values. And I managed to remove this restriction from the driver.

So here is a test ensuring this behavior.