Fix sending iTIP counter proposal generating invalid messages

First, a recent regression has caused the counter proposals to be sent with REQUEST method rather than COUNTER. Second, even when the correct value of the METHOD property in the iCal was set, the "method" parameter in the Content-Type header of the iTIP mime part was still hard-coded to REQUEST. This change makes sure that we conform to the iMIP RFC and always set the same method both in the header and inside the iCal invitation.