Wrong submit the media browser form in iframe with "enter" key.

Hi everyone.

If you create a field with a mediabrowser widget and will try to filter files there. But instead of clicking on "Filter" button will press "enter" on your keyboard then the form will be submitted and redirected to /admin/content/media_browser, so all the menus will be shown and other unnecessary html. To prevent it I've created a small patch.

Hope it'll help someone.

Actually the same problem happens in 7.x-1.x-dev version.