Issue with dict_of_key_value_pairs on Python < 2.7

If you pass a unicode string to `dict_of_key_value_pairs` in Python < 2.7, it raises a `ValueError` because of an issue in shlex (probably this one http://bugs.python.org/issue1170)

Here's a test to reproduce the bug.

Two fix ideas:

* encode unicode strings in `dict_of_key_value_pairs` (utf-8 works fine here)
* assert that the argument must not be of type `unicode` in Python < 2.7