Send out megolm keys when we start a megolm session

For now, pending the arrival of SPEC-138, this happens via inline messages in
the room.

lib/crypto-algorithms/olm.js

@@ -56,17 +56,13 @@ OlmEncryption.prototype.initRoomEncryption = function(roomMembers) {
 /**
  * @inheritdoc
  *
- * @param {module:models/room?} room
+ * @param {module:models/room} room
  * @param {string} eventType
  * @param {object} plaintext event content
  *
  * @return {module:client.Promise} Promise which resolves to the new event body
  */
 OlmEncryption.prototype.encryptMessage = function(room, eventType, content) {
-    if (!room) {
-        throw new Error("Cannot send encrypted messages in unknown rooms");
-    }
-
     // pick the list of recipients based on the membership list.
     //
     // TODO: there is a race condition here! What if a new user turns up