base: Set response charset

WebOb has started raising an exception when writing a Unicode string to
`Response` objects if no charset is set.

src/dcow/base.py

@@ -30,10 +30,12 @@ class VariedResponse(milla.Response):
             context = {}
         if template is None or self.request.want == 'json':
             self.content_type = str('application/json')
+            self.charset = 'utf-8'
             json.dump(context, self.body_file)
         else:
             if self.request.want == 'xhtml':
                 self.content_type = str('application/xhtml+xml')
+            self.charset = 'utf-8'
             self.render(template, context)
 
     def render(self, template, context=None):