Fix saving EPS to open file objects on Python 2.x

Fix issues #470 and #479.
Maybe @fluggo can review this PR and commit https://github.com/python-imaging/Pillow/commit/4f7d784a7137c4a0f7c75891a63c5f0cd9170dd0 for a better fix.
As mentioned before https://github.com/python-imaging/Pillow/issues/470#issuecomment-31677359, testing for EPS is spotty...