lint: remove deprecated String.prototype.substr

closes #459

Author: CommanderRoot

lib/types/json.js

@@ -122,7 +122,7 @@ function json (options) {
 
     // assert charset per RFC 7159 sec 8.1
     var charset = getCharset(req) || 'utf-8'
-    if (charset.substr(0, 4) !== 'utf-') {
+    if (charset.slice(0, 4) !== 'utf-') {
       debug('invalid charset')
       next(createError(415, 'unsupported charset "' + charset.toUpperCase() + '"', {
         charset: charset,