Fixed #15679 - regression in HttpRequest.POST and raw_post_data access.

Thanks to vkryachko for the report.

This also fixes a slight inconsistency with raw_post_data after parsing of a
multipart request, and adds a test for that.  (Previously accessing
raw_post_data would have returned the empty string rather than raising an
Exception).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15938 bcc190cf-cafb-0310-a4f2-bffc1f526a37