Make interception work with redirects (#218)

This patch:
- changes interception API so that it better aligns with what we'd like to see
  in #121
- fixes the issue with redirect interception

Fixes #217.

lib/NetworkManager.js

@@ -247,17 +247,23 @@ class InterceptedRequest {
     });
   }
 
-  continue() {
+  /**
+   * @param {!Object} overrides
+   */
+  continue(overrides = {}) {
     console.assert(!this._handled, 'This request is already handled!');
     this._handled = true;
-    let headers = {};
-    for (let entry of this.headers.entries())
-      headers[entry[0]] = entry[1];
+    let headers = undefined;
+    if (overrides.headers) {
+      headers = {};
+      for (let entry of overrides.headers.entries())
+        headers[entry[0]] = entry[1];
+    }
     this._client.send('Network.continueInterceptedRequest', {
       interceptionId: this._interceptionId,
-      url: this.url,
-      method: this.method,
-      postData: this.postData,
+      url: overrides.url,
+      method: overrides.method,
+      postData: overrides.postData,
       headers: headers
     });
   }