protocol decoder: add request reader factory method (#34)

Github Issue: #34

Add the factory method to initial decoder to encapsulate the
initialization parameters of request reader those will be added after
this.

lib/rims/protocol/decoder.rb

@@ -85,7 +85,7 @@ def self.repl(decoder, limits, input, output, logger)
         apply_imap_command.call(:ok_greeting)
 
         conn_timer = ConnectionTimer.new(limits, input.to_io)
-        request_reader = RequestReader.new(input, output, logger)
+        request_reader = decoder.make_requrest_reader(input, output)
 
         until (conn_timer.command_wait_timeout?)
           conn_timer.command_wait or break
@@ -341,6 +341,10 @@ def initialize(drb_services, auth, logger,
         @logger.debug("RIMS::Protocol::InitialDecoder#initialize at #{self}") if @logger.debug?
       end
 
+      def make_requrest_reader(input, output)
+        RequestReader.new(input, output, @logger)
+      end
+
       def auth?
         false
       end