Implement Request.setHeader() method

This patch implements Request.setHeader() method to override
request headers.

References #8.

lib/Request.js

@@ -71,10 +71,13 @@ class Request {
     }
 
     /**
-     * @param {!Object} headers
+     * @param {string} key
+     * @param {string} value
      */
-    setHeaders(headers) {
-        this._headersOverride = headers;
+    setHeader(key, value) {
+        if (!this._headersOverride)
+            this._headersOverride = Object.assign({}, this._headers);
+        this._headersOverride[key] = value;
     }
 
     /**

third_party/phantomjs/test/module/webpage/add-header.js

@@ -1,4 +1,3 @@
-//! unsupported
 async_test(function () {
     var webpage = require('webpage');