require 'debug' doesn't change SIGINT

`parse_config_value` only check String.

ruby#148 (comment)

Author: ko1

lib/debug/config.rb

@@ -66,7 +66,7 @@ def self.config_to_env_hash config
   end
 
   def self.parse_config_value name, valstr
-    return nil if valstr == nil
+    return valstr unless valstr.kind_of? String
 
     case CONFIG_SET[name][2]
     when :bool