properly quote www-authenticate header value

HTTP header values should be quoted. Since the WWW-Authenticate
header value contains user-supplied strings, it's important to
ensure it's properly quoted to ensure the integrity of the protocol.

Previous to this patch, the URL was unquoted and then the unquoted
value was returned in the header. This patch re-quotes the value
when it is set on the response.

This is filed as CVS-2014-3497

Fixes bug 1327414

Change-Id: If8bd8842f2ce821756e9b4461a18a8ac8d42fb8c
(cherry picked from commit b223322ed1ef44f61490f820240aa01f1047ae2e)