Make sure all variables passed to request are str

All parameters passed to request should be bytestrings - str. If any
of them is accidentally unicode, this will try to convert all bytestrings
to unicode via str.decode(sys.defaultencoding). This is problematic,
because non-ascii data will then throw an UnicodeDecodeError as shown in [1].

[1] http://lists.wikimedia.org/pipermail/pywikipedia-l/2013-August/008218.html

Change-Id: I1b66a31c8752dee1f950d425d532479f1b671926
Merge "Make sure all variables passed to request are str"