[1.3.X] Fixed #10571 -- Factored out the payload encoding code to make sure it...

[1.3.X] Fixed #10571 -- Factored out the payload encoding code to make sure it is used for PUT requests. Thanks to kennu for the report, pterk for the patch, and wildfire for the review comments.

Backport of r16651 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16672 bcc190cf-cafb-0310-a4f2-bffc1f526a37