Fix an obscure issue in one of the tests related to Windows, older versions of Python, and float('nan')

Conflicts:

	lib/pyfits/tests/test_header.py


Conflicts:

	lib/pyfits/tests/test_header.py

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.0-stable@2360 ed100bfc-0583-0410-97f2-c26b58777a21