Drop Headers class in favor of a Map object

This patch removes Header class and substitutes it with a simple
Map object.

The map is chosen over the vanilla object since it has explicit
order of headers which we'd like to preserve.

References #106.

lib/NetworkManager.js

@@ -136,91 +136,6 @@ class NetworkManager extends EventEmitter {
   }
 }
 
-class Headers {
-  /**
-   * @param {?Object} payload
-   * @return {!Headers}
-   */
-  static fromPayload(payload) {
-    let headers = new Headers();
-    if (!payload)
-      return headers;
-    for (let key in payload)
-      headers.set(key, payload[key]);
-    return headers;
-  }
-
-  constructor() {
-    /** @type {!Map<string, string>} */
-    this._headers = new Map();
-  }
-
-  /**
-   * @param {string} name
-   * @param {string} value
-   */
-  append(name, value) {
-    name = name.toLowerCase();
-    this._headers.set(name, value);
-  }
-
-  /**
-   * @param {string} name
-   */
-  delete(name) {
-    name = name.toLowerCase();
-    this._headers.delete(name);
-  }
-
-  /**
-   * @return {!Iterator}
-   */
-  entries() {
-    return this._headers.entries();
-  }
-
-  /**
-   * @param {string} name
-   * @return {?string}
-   */
-  get(name) {
-    name = name.toLowerCase();
-    return this._headers.get(name);
-  }
-
-  /**
-   * @param {string} name
-   * @return {boolean}
-   */
-  has(name) {
-    name = name.toLowerCase();
-    return this._headers.has(name);
-  }
-
-  /**
-   * @return {!Iterator}
-   */
-  keys() {
-    return this._headers.keys();
-  }
-
-  /**
-   * @return {!Iterator}
-   */
-  values() {
-    return this._headers.values();
-  }
-
-  /**
-   * @param {string} name
-   * @param {string} value
-   */
-  set(name, value) {
-    name = name.toLowerCase();
-    this._headers.set(name, value);
-  }
-}
-
 class Body {
   /**
    * @param {function():!Promise<!Buffer>} contentCallback
@@ -282,7 +197,7 @@ class Request extends Body {
     this._response = null;
     this.url = payload.url;
     this.method = payload.method;
-    this.headers = Headers.fromPayload(payload.headers);
+    this.headers = new Map(Object.entries(payload.headers));
   }
 
   /**
@@ -303,7 +218,7 @@ class Response extends Body {
   constructor(request, payload, contentCallback) {
     super(contentCallback);
     this._request = request;
-    this.headers = Headers.fromPayload(payload.headers);
+    this.headers = new Map(Object.entries(payload.headers));
     this.ok = payload.status >= 200 && payload.status <= 299;
     this.status = payload.status;
     this.statusText = payload.statusText;
@@ -332,7 +247,7 @@ class InterceptedRequest {
 
     this.url = payload.url;
     this.method = payload.method;
-    this.headers = Headers.fromPayload(payload.headers);
+    this.headers = new Map(Object.entries(payload.headers));
     this.postData = payload.postData;
   }