Allow TestAgent pass a cert and key to request (ladjs#373)

* Update agent.js with superagent options

Superagent added the ability to set the SSL certificate and key parameters for requests, so they need to be added to supertest in order to pass them through to superagent.

* Cleaning up slightly

lib/agent.js

@@ -25,7 +25,11 @@ module.exports = TestAgent;
 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;
+  if (options) {
+    this._ca = options.ca;
+    this._key = options.key;
+    this._cert = options.cert;
+  }
   Agent.call(this);
   this.app = app;
 }
@@ -41,6 +45,8 @@ methods.forEach(function(method) {
   TestAgent.prototype[method] = function(url, fn) {   // eslint-disable-line no-unused-vars
     var req = new Test(this.app, method.toUpperCase(), url);
     req.ca(this._ca);
+    req.cert(this._cert);
+    req.key(this._key);
 
     req.on('response', this._saveCookies.bind(this));
     req.on('redirect', this._saveCookies.bind(this));