don't use http2 agent when url is http (#76)

lib/agent.js

@@ -16,12 +16,16 @@ import { parse } from 'url';
 export default class Agent {
   constructor (url, { debug } = {}) {
     this._url = url;
-    this._host = parse(url).host;
+    const { protocol, host } = parse(url);
+    this._protocol = protocol;
+    this._host = host;
     this._debug = debug;
     this._initAgent();
   }
 
   _initAgent () {
+    if ('https:' !== this._protocol) return;
+
     this._agent = http2.createAgent({
       host: this._host,
       port: 443
@@ -60,6 +64,6 @@ export default class Agent {
 
   close () {
     if (this._debug) console.log('> [debug] closing agent');
-    return this._agent.close();
+    if (this._agent) return this._agent.close();
   }
 }