Fix generating form data for IntegerFields.

For example Django's FormSet ManagementForm. It's not legal to put non-strings
(ints, floats, etc) into GET or POST data for fast_dispatch, so everything
needs to be converted to a string first.
Fix generating form data for formsets.

We need to take the form's prefix into account when generating the POST data.
This breaks backwards compatibility; for each formset you must prefix any data
that you're supplying explicitly with the form's prefix.