[1.2.X] Ensure render_to_string leaves the context instance stack in the state...

[1.2.X] Ensure render_to_string leaves the context instance stack in the state it was originally passed in.

Backport of r15591 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15592 bcc190cf-cafb-0310-a4f2-bffc1f526a37