Fix arity change on Resque::Failure.clear

Resque::Failure::Redis was not updated to reflect this change

Author: tarcieri

lib/resque/failure/redis.rb

@@ -37,7 +37,8 @@ def self.each(offset = 0, limit = self.count, queue = :failed)
         end
       end
 
-      def self.clear
+      def self.clear(queue = nil)
+        raise ArgumentError, "invalid queue: #{queue}" if queue && queue.to_s == "failed"
         Resque.redis.del(:failed)
       end