[artwork/http] Check response code and content type

Extends the http_client_ctx to hold the response code for a request.
Also adds the content type header, if it was a https request (using
libcurl instead of libevent)

src/http.h

@@ -29,6 +29,9 @@ struct http_client_ctx
    */
   int headers_only;
 
+  /* HTTP Response code */
+  int response_code;
+
   /* Private */
   int ret;
   void *evbase;