Pass watch: kwarg to #multi calls on clients

Both RedisClient and RedisClient::Cluster have #multi methods that
accept a watch: kwarg to watch keys as part of the transaction. Expose
that argument with the wrapped subclass.

Author: KJ Tsanaktsidis

cluster/lib/redis/cluster/client.rb

@@ -70,8 +70,8 @@ def pipelined(&block)
         handle_errors { super(&block) }
       end
 
-      def multi(&block)
-        handle_errors { super(&block) }
+      def multi(watch: nil, &block)
+        handle_errors { super(watch: watch, &block) }
       end
 
       private

lib/redis/client.rb

@@ -111,7 +111,7 @@ def pipelined
       Client.translate_error!(error)
     end
 
-    def multi
+    def multi(watch: nil)
       super
     rescue ::RedisClient::Error => error
       Client.translate_error!(error)