hopefulles solves #909
fix typo
Fix failure when adding a row to an empty masked Table

Also fixed another issue where (with masked table) calling
add_row() would add a row copying the first row instead of the
advertised behavior of adding a row of "zeros".  The docstring
was clear that it should be zeros.
Merge branch 'fix909' of github.com:hamogu/astropy into fix909
Improve according to suggestions in discussion on #909
This should fix issue #968; this handles the situation in the following way:

If opening a file in a writeable mode a warning is raised and mmap is
disabled even if it was explicitly asked for.  This is done through a
new check that mmap.flush() is available--this check is performed on all
platforms and should require minimal overhead but we'll see... (the
check is only performed the first time an attempt is made to open a file
in writeable mode)
Added a note in the known issues page and a changelog entry
hopefulles solves #909
fix typo
Improve according to suggestions in discussion on #909
Fix syntax issue on Python 3
Add missing byte literal marker needed for Python 3 (since mkstemp automatically opens the file in binary mode)
Merge remote-tracking branch 'github/fix909' into fix909
update tests to return string

Also, I added a check to test if format functions return a string (only
once, when it is called first). This will be a common source of error
and this test allows us to provide a meaningful error message
for 95% of all cases.
Update known_issues.rst

Made more explicit that the Hurd mmap issue relates to FITS files specifically.
Raise a big fat exception if creating a unit with the same name as an existing unit.
Merge pull request #1070 from mdboom/unit/exception-on-redefine

Raise exception if creating a unit with the same name as an existing unit
Update file.py

se -> set
Merge pull request #968 from iguananaut/issue-968

Test failure on Debian/Hurd
do changes suggested by @taldcroft in the discussion of #909