Decode request.query_string before use.

Decode request.query_string before use.

This doesn't seem to be an issue in my Guix SD/Python 3.7 testing, but hit an
issue on Debian 9/Python3.5 where concatenating text to `request.query_string`
was causing the error:

  TypeError: a bytes-like object is required, not 'str'

This clearly hasn't been an issue in the past though, so I suspect there's a
library somewhere that's on a different version and is behaving differently.