Auth token, set the correct charset

Explicit set the charset to UTF-8 when replying with 401. It's now
required by webob.

A previous attempt to fix this problem was introduced as part of the
commit 20fb1dbe5a884ecdbf6ee5ea26b2660f7a7e4b1e but this is only
compatible with the version 1.7.0 of WebOb, older versions will break.
See:
http://logs.openstack.org/05/371605/12/check/gate-ironic-python35-db/686bd86/console.html#_2017-01-03_11_10_11_760197

Change-Id: I30adc87882ce8bd2c18588161d5c60d77a56925e
Closes-Bug: #1653646

keystonemiddleware/auth_token/__init__.py

@@ -651,7 +651,8 @@ class AuthProtocol(BaseAuthProtocol):
                 raise webob.exc.HTTPUnauthorized(
                     body=jsonutils.dumps(body),
                     headers=self._reject_auth_headers,
-                    content_type='application/json; charset=UTF-8')
+                    charset='UTF-8',
+                    content_type='application/json')
 
         if request.user_token_valid:
             request.set_user_headers(request.token_auth.user)