Work around some tets that fail on Windows due to floating point differences

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2490 ed100bfc-0583-0410-97f2-c26b58777a21
Fixed a very subtle bug in the handling of zero-width columns, related to the fact that a column's index in the list of columns described by the FITS header is different from its index in the recarray holding the table's data (which excludes any zero-width columns).  This was actually 'revealed' by making encode_ascii and decode_ascii a little less careless in how it passes through non-string arrays.

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2491 ed100bfc-0583-0410-97f2-c26b58777a21