Comment 3 for bug 891186

That fix assumes all values will be unicode - something that isn't (currently) guaranteed. referrer is probably (or should be) bytestrings [its an http header: they are bytestrings on the wire, and for all upstream python posturing, a unicode http header is ill defined; particularly as there is now a bis recommendation for how to put unicode into a header: headers that are badly encoded will still exist].

Essentially we have a mix of data sources, so I think this needs to be type checking.