feat(TestAgent): decoupled TestAgent and superagent's Agent to support more than ca, key, cert

`options` object is now a direct passthrough, TestAgent can support whichever superagent's Agent is expecting
i.e. `ca`, `key`, `cert`, `pfx`, `rejectUnauthorized`

Author: lamweili

Diff for: lib/agent.js

@@ -20,12 +20,7 @@ const Test = require('./test.js');
 function TestAgent(app, options) {
   if (!(this instanceof TestAgent)) return new TestAgent(app, options);
   if (typeof app === 'function') app = http.createServer(app); // eslint-disable-line no-param-reassign
-  if (options) {
-    this._ca = options.ca;
-    this._key = options.key;
-    this._cert = options.cert;
-  }
-  Agent.call(this);
+  Agent.call(this, options);
   this.app = app;
 }
 
@@ -45,9 +40,7 @@ TestAgent.prototype.host = function(host) {
 methods.forEach(function(method) {
   TestAgent.prototype[method] = function(url, fn) { // eslint-disable-line no-unused-vars
     const req = new Test(this.app, method.toUpperCase(), url, this._host);
-    req.ca(this._ca);
-    req.cert(this._cert);
-    req.key(this._key);
+
     if (this._host) {
       req.set('host', this._host);
     }