[1.0.X] Fixed #8817: get_image_dimensions correctly closes the files it opens,...

[1.0.X] Fixed #8817: get_image_dimensions correctly closes the files it opens, and leaves open the ones it doesn't. Thanks, mitsuhiko.

While I was at it, I converted the file_storage doctests to unit tests.

Backport of [10707] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10708 bcc190cf-cafb-0310-a4f2-bffc1f526a37