Initial support for specifying which servers to try in joinRoom

This has a bug when using browser-request where the query string for `server_name: [a, b]` comes out as `?server_name=a,b` instead of `?server_name=a&server_name=b`. This is due to browser-request not supporting the same qs options as request, so the qsStringifyOptions do nothing.

src/client.js

@@ -921,6 +921,8 @@ MatrixClient.prototype.isUserIgnored = function(userId) {
  * </strong> Default: true.
  * @param {boolean} opts.inviteSignUrl If the caller has a keypair 3pid invite,
  *                                     the signing URL is passed in this parameter.
+ * @param {string[]} opts.viaServers The server names to try and join through in
+ *                                   addition to those that are automatically chosen.
  * @param {module:client.callback} callback Optional.
  * @return {module:client.Promise} Resolves: Room object.
  * @return {module:http-api.MatrixError} Rejects: with an error response.
@@ -949,6 +951,13 @@ MatrixClient.prototype.joinRoom = function(roomIdOrAlias, opts, callback) {
         );
     }
 
+    const queryString = {};
+    if (opts.viaServers) {
+        queryString["server_name"] = opts.viaServers;
+    }
+
+    const reqOpts = {qsStringifyOptions: {arrayFormat: 'repeat'}};
+
     const defer = Promise.defer();
 
     const self = this;
@@ -959,7 +968,7 @@ MatrixClient.prototype.joinRoom = function(roomIdOrAlias, opts, callback) {
         }
 
         const path = utils.encodeUri("/join/$roomid", { $roomid: roomIdOrAlias});
-        return self._http.authedRequest(undefined, "POST", path, undefined, data);
+        return self._http.authedRequest(undefined, "POST", path, queryString, data, reqOpts);
     }).then(function(res) {
         const roomId = res.room_id;
         const syncApi = new SyncApi(self, self._clientOpts);