Make sure that charset attribute is a str.

Changed:
quixote/http_response.py

Here is the diff:
Index: quixote/http_response.py
===================================================================
--- quixote/http_response.py	(revision 26250)
+++ quixote/http_response.py	(working copy)
@@ -165,7 +165,7 @@
         self.content_type = content_type
 
     def set_charset(self, charset):
-        self.charset = charset.lower()
+        self.charset = str(charset).lower()
 
     def set_status(self, status, reason=None):
         """set_status(status : int, reason : string = None)