Fixes spacetelescope/PyFITS#7. Also fixed a tangential bug with using Header.index() on HIERARCH keywords that was caught in the implementation of the test for the original issue.

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.1-stable@2154 ed100bfc-0583-0410-97f2-c26b58777a21