Make namespace symbol friendly

Currently limit_fetch fails to work with symbol namespaces with following
error: `Error fetching job: undefined method `+' for :sidekiq:Symbol`

lib/sidekiq/limit_fetch/queues.rb

@@ -48,7 +48,7 @@ module Sidekiq::LimitFetch::Queues
   def namespace
     @namespace ||= Sidekiq.redis do |it|
       if it.respond_to?(:namespace) and it.namespace
-        it.namespace + ':'
+        "#{it.namespace}:"
       else
         ''
       end