Field UI redirections broken

When you create a new field, you're supposed to go through the 'field settngs' page and the 'instance + widget settings' page.
The second redirection didn't work anymore after #216098: drupal_goto() does not follow same parameters as url() (changed drupal_goto() param format).

Simple fix, should be RTBC.

Note: corresponding tests are being worked on at #600544: Tests for Field UI. I suggest we commit this fix so that the tests can check the correct behavior.