Making build_statement not modify the given argument

Because improves readability and code quality.

Co-authored-by: tomoya ishida <tomoyapenguin@gmail.com>

Author: tikkss

lib/irb.rb

@@ -1132,7 +1132,7 @@ def build_statement(code)
         return Statement::EmptyInput.new
       end
 
-      code.dup.force_encoding(@context.io.encoding)
+      code = code.dup.force_encoding(@context.io.encoding)
       if (command, arg = @context.parse_command(code))
         command_class = Command.load_command(command)
         Statement::Command.new(code, command_class, arg)