Fix error "No sentinels available" when the master name is different from the host name

Author: NguyenDucTung

lib/redis/client.rb

@@ -493,7 +493,7 @@ def initialize(options)
 
           @sentinels = @options.delete(:sentinels).dup
           @role = @options.fetch(:role, "master").to_s
-          @master = @options[:host]
+          @master = @options[:master_name] || @options[:host]
         end
 
         def check(client)