Use instance_variable_defined? to avoid warning

This avoids Ruby complaining that we're referencing the @Paginator instance variable which may not be defined at this point.

Author: ekampp

lib/api-pagination/configuration.rb

@@ -50,7 +50,11 @@ def initialize
     end
 
     def paginator
-      @paginator || set_paginator
+      if instance_variable_defined? :@paginator
+        @paginator
+      else
+        set_paginator
+      end
     end
 
     def paginator=(paginator)