Fixed #10645 -- Added some robustness around some admin and modelform params.

Fieldset dictionary names, search fields and unique_together attribute
names all have to be convertible to strings (that has always been true).
If somebody passes in a unicode object, Python barfs because Django uses
those values as keyword argument names and function calls require
parameter names to be str objects. We now convert thing to strs
automatically.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10510 bcc190cf-cafb-0310-a4f2-bffc1f526a37