Coerce keyword args to str for 2.6 <= Python < 2.6.6

Releases of Python 2.6 before 2.6.6 raise TypeError if given keyword args
where the keywords are in Unicode.  When code imports unicode_literals
from ``__future__``, even keywords that use only ASCII characters can trigger
this problem (when they are inserted as Unicode strings into a kwargs dict).

See http://cuu508.wordpress.com/2011/01/27/keywords-must-be-strings/ and
https://github.com/jsocol/scrumbugz/commit/e12238c28 for other examples.