Fixed a typo in the previous commit that caused a crash when comparing header values.  Fixed another issue where diffing the data after diffing the headers could cause the headers to change if the data was scaled--mitigated by keeping a copy of the headers before the data is accessed.
Fixed a typo in the previous commit that caused a crash when comparing header values.  Fixed another issue where diffing the data after diffing the headers could cause the headers to change if the data was scaled--mitigated by keeping a copy of the headers before the data is accessed.

git-svn-id: 5305e2c1a78737cf7dd5f8f44e9bbbd00348fde7@1598 ed100bfc-0583-0410-97f2-c26b58777a21

spacetelescope/PyFITS,embray/PyFITS,embray/PyFITS,spacetelescope/PyFITS,embray/PyFITS,embray/PyFITS