hopefulles solves #909
fix typo
Merge branch 'fix909' of github.com:hamogu/astropy into fix909
Improve according to suggestions in discussion on #909
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.