Fix a subtle Unicode problem in Response.assemble

If msg is Unicode, the proto string is automatically promoted to Unicode.  If
the proto string is promoted to Unicode, then the FMT interpolation is also
done in Unicode. If this happens, then binary data in content will cause an
exception.

libmproxy/proxy.py

@@ -338,7 +338,7 @@ class Response(controller.Msg):
             content = ""
         if self.request.client_conn.close:
             headers["connection"] = ["close"]
-        proto = "HTTP/1.1 %s %s"%(self.code, self.msg)
+        proto = "HTTP/1.1 %s %s"%(self.code, str(self.msg))
         data = (proto, str(headers), content)
         return self.FMT%data