Fixed #16138 -- Made FormMixin get_initial return a copy of the 'initial' class variable. Thanks hanson2010, wilfred@potatolondon.com and agriffis for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17765 bcc190cf-cafb-0310-a4f2-bffc1f526a37