Adds a signal handler to @msg for @signal, as with
g_signal_connect(), but the @callback will only be run if @msg's
incoming messages headers (that is, the
<literal>request_headers</literal> for a client #SoupMessage, or
the <literal>response_headers</literal> for a server #SoupMessage)
contain a header named @header.
Adds a signal handler to @msg for @signal, as with g_signal_connect(), but the @callback will only be run if @msg's incoming messages headers (that is, the <literal>request_headers</literal> for a client #SoupMessage, or the <literal>response_headers</literal> for a server #SoupMessage) contain a header named @header.