chore: use the ability of `content-type` lib directly (#1276)

lib/request.js

@@ -367,16 +367,12 @@ module.exports = {
    */
 
   get charset() {
-    let type = this.get('Content-Type');
-    if (!type) return '';
-
     try {
-      type = contentType.parse(type);
+      const { parameters } = contentType.parse(this.req);
+      return parameters.charset || '';
     } catch (e) {
       return '';
     }
-
-    return type.parameters.charset || '';
   },
 
   /**