Include the HTTP "WWW-Authenticate" response header field

[dmitrykrivaltsevich]

RFC 6750 defines response headers for various cases. In the section 3 it states:

   If the protected resource request does not include authentication
   credentials or does not contain an access token that enables access
   to the protected resource, the resource server MUST include the HTTP
   "WWW-Authenticate" response header field; it MAY include it in
   response to other conditions as well.  The "WWW-Authenticate" header
   field uses the framework defined by HTTP/1.1 [RFC2617].

So we need to add this header in order to make the library more compliant with the standard.