Implement Request object

This patch does a step towards Fetch API:
- implements Request object to some extend. The Request object will be
  sent in RequestWillBeSent event.
- implements InterceptedRequest which extends from Request and allows
  for request modification. The InterceptedRequest does not
  conform to Fetch API spec - there seems to be nothing related to
  amending in-flight request.
- adds test to make sure that request can change headers.

References #26

lib/Page.js

@@ -17,7 +17,7 @@
 let fs = require('fs');
 let EventEmitter = require('events');
 let mime = require('mime');
-let Request = require('./Request');
+let {InterceptedRequest} = require('./Request');
 let Navigator = require('./Navigator');
 let Dialog = require('./Dialog');
 let FrameManager = require('./FrameManager');
@@ -57,7 +57,7 @@ class Page extends EventEmitter {
     this._extraHeaders = {};
     /** @type {!Map<string, function>} */
     this._inPageCallbacks = new Map();
-    /** @type {?function(!Request)} */
+    /** @type {?function(!InterceptedRequest)} */
     this._requestInterceptor = null;
     /** @type {?Promise<number>} */
     this._rootNodeIdPromise = null;
@@ -94,7 +94,7 @@ class Page extends EventEmitter {
   }
 
   /**
-   * @param {?function(!Request)} interceptor
+   * @param {?function(!InterceptedRequest)} interceptor
    */
   async setRequestInterceptor(interceptor) {
     this._requestInterceptor = interceptor;
@@ -105,7 +105,7 @@ class Page extends EventEmitter {
    * @param {!Object} event
    */
   _onRequestIntercepted(event) {
-    let request = new Request(this._client, event.InterceptionId, event.request);
+    let request = new InterceptedRequest(this._client, event.InterceptionId, event.request);
     this._requestInterceptor(request);
   }