Fix logical error in legacy config expression

We should check for an undefined and not just for a logical true or
false.

Example: When `usecdn` was set to false it was impossible to overwrite
the new config value because the if statement becomes false.

Thanks @davidmehren for pointing me to this issue.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

lib/config/index.js

@@ -110,8 +110,8 @@ for (let i = keys.length; i--;) {
   // and the config with uppercase is not set
   // we set the new config using the old key.
   if (uppercase.test(keys[i]) &&
-  config[lowercaseKey] &&
-  !config[keys[1]]) {
+  config[lowercaseKey] !== undefined &&
+  fileConfig[keys[i]] === undefined) {
     logger.warn('config.js contains deprecated lowercase setting for ' + keys[i] + '. Please change your config.js file to replace ' + lowercaseKey + ' with ' + keys[i])
     config[keys[i]] = config[lowercaseKey]
   }