fixing gzip import issue 2774
Code now import both astropy and system gzip

For clarity the astropy gzip is called _astropy_gzip
and the system gzip is called _system_gzip.

All gzip.x statements have been changed to use the
_astropy_gzip.

All isinstances statements check both
creating global variable with the two GzipFile types
Added test for gzip file suggested by Erik Bray.
adding issue 2783 to the changelog for 0.4.2
Merge pull request #2783 abostroem/gzip_fix