Changed the (internal) way extra(select=.., select_params=...) handling is done

so that parameters stay with their select items. This means that merging and
trimming of those items is handled correctly.

Refs #7957, #7961. Fixed #8191.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8426 bcc190cf-cafb-0310-a4f2-bffc1f526a37