Corrected change in behavior regarding the page shown after the 'Save' button...

Corrected change in behavior regarding the page shown after the 'Save' button is pressed when adding a user through the admin.

It had been introduced in trunk (r13503) and between 1.2.1 and 1.2.2 (r13504). The original fix intended to correct a similar problem introduced between 1.1 and 1.2 (r12218) this time in the 'Save and add another' button.
We have now tests for the three buttons present in the Add User admin form to avoid future regressions.
Thanks to Juan Pedro Fisanotti and Cesar H. Roldan for their work.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14628 bcc190cf-cafb-0310-a4f2-bffc1f526a37