[1.3.X] Fixed #17634 -- Optimized the performance of MultiValueDict by using...

[1.3.X] Fixed #17634 -- Optimized the performance of MultiValueDict by using append instead of copy and by minimizing the number of dict lookups. Backport of r17464 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17807 bcc190cf-cafb-0310-a4f2-bffc1f526a37