Rename page.setHTTPHeaders into page.setExtraHTTPHeaders

This patch:
- renames page.setHTTPHeaders into page.setExtraHTTPHeaders
- starts using Map instead of Object to align with other headers
  arguments

Fixes #112.

lib/NetworkManager.js

@@ -24,9 +24,10 @@ class NetworkManager extends EventEmitter {
     super();
     this._client = client;
     this._requestInterceptor = null;
-    /* @type {!Map<string, !Request>} */
+    /** @type {!Map<string, !Request>} */
     this._idToRequest = new Map();
-    this._httpHeaders = {};
+    /** @type {!Map<string, string>} */
+    this._extraHTTPHeaders = new Map();
 
     this._client.on('Network.requestWillBeSent', this._onRequestWillBeSent.bind(this));
     this._client.on('Network.requestIntercepted', this._onRequestIntercepted.bind(this));
@@ -36,22 +37,22 @@ class NetworkManager extends EventEmitter {
   }
 
   /**
-   * @param {!Object} headers
+   * @param {!Map<string, string>} extraHTTPHeaders
    * @return {!Promise}
    */
-  async setHTTPHeaders(headers) {
-    this._httpHeaders = {};
-    // Note: header names are case-insensitive.
-    for (let key of Object.keys(headers))
-      this._httpHeaders[key.toLowerCase()] = headers[key];
-    return this._client.send('Network.setExtraHTTPHeaders', { headers });
+  async setExtraHTTPHeaders(extraHTTPHeaders) {
+    this._extraHTTPHeaders = new Map(extraHTTPHeaders);
+    let headers = {};
+    for (let entry of extraHTTPHeaders.entries())
+      headers[entry[0]] = entry[1];
+    await this._client.send('Network.setExtraHTTPHeaders', { headers });
   }
 
   /**
-   * @return {!Object}
+   * @return {!Map<string, string>}
    */
-  httpHeaders() {
-    return Object.assign({}, this._httpHeaders);
+  extraHTTPHeaders() {
+    return new Map(this._extraHTTPHeaders);
   }
 
   /**