Expand documentation for fill_values

fixes #2080
[skip ci]
Fix typo [skip ci]
Merge pull request #2904 from larrybradley/typo

Fix typo [skip ci]
Merge pull request #2902 from hamogu/fix2080_2

Expand documentation for fill_values
modified changelog to be more specific about fixing gzip when Tables is called
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
adding issue 2783 to the changelog for 0.4.1
fixing indentation of global variable
adding test for gzip. This test fails in astropy versions 0.4 and lower and passes for 0.4.1 and higher
Added test for gzip file suggested by Erik Bray.

This tests the utils.compat.gzip function's ability to open files
this test has been incorporated into test_core.py
removing trailing whitespace from the new test
removing last trailing whitespace
modified changelog to be more specific about fixing gzip when Tables is called
Merge branch 'gzip_fix'