A test and fix for spacetelescope/PyFITS#21

Conflicts:

	lib/pyfits/tests/test_diff.py

Conflicts:

	CHANGES.txt
	lib/pyfits/tests/test_diff.py

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.1-stable@2537 ed100bfc-0583-0410-97f2-c26b58777a21