Fix TypeError: unicode argument expected, got 'str'

[lpramuk]

Root cause:

$ python2
Python 2.7.14 (default, Dec 11 2017, 16:08:01) 
[GCC 7.2.1 20170915 (Red Hat 7.2.1-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from io import StringIO
>>> StringIO().write('str')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unicode argument expected, got 'str'
>>> StringIO().write(u'str')
3L

I switched to start using StringIO from io and forgot to add u'str'

[JacobCallahan]

ACK, Thanks!

[JacobCallahan]

@lpramuk if you can't get any more reviewers in the next 30mins, I'll check back in and merge this.

Fixes #654