Get rid of Body class

The Body class was inlined in the Request and Response classes.
This patch:
- removes the Body class
- adds Request.postData public property
- adds Response.buffer(), Response.text() and Response.json() methods

Fixes #106.

lib/NetworkManager.js

@@ -137,14 +137,42 @@ class NetworkManager extends EventEmitter {
   }
 }
 
-class Body {
+class Request {
   /**
+   * @param {!Object} payload
+   */
+  constructor(payload) {
+    this._response = null;
+    this.url = payload.url;
+    this.method = payload.method;
+    this.postData = payload.postData;
+    this.headers = new Map(Object.entries(payload.headers));
+  }
+
+  /**
+   * @return {?Response}
+   */
+  response() {
+    return this._response;
+  }
+}
+helper.tracePublicAPI(Request);
+
+class Response {
+  /**
+   * @param {?Request} request
+   * @param {!Object} payload
    * @param {function():!Promise<!Buffer>} contentCallback
    */
-  constructor(contentCallback) {
+  constructor(request, payload, contentCallback) {
+    this._request = request;
     this._contentCallback = contentCallback;
-    /** @type {?Promise<!Buffer>} */
     this._contentPromise = null;
+    this.headers = new Map(Object.entries(payload.headers));
+    this.ok = payload.status >= 200 && payload.status <= 299;
+    this.status = payload.status;
+    this.statusText = payload.statusText;
+    this.url = payload.url;
   }
 
   /**
@@ -171,45 +199,6 @@ class Body {
     let content = await this.text();
     return JSON.parse(content);
   }
-}
-helper.tracePublicAPI(Body);
-
-class Request extends Body {
-  /**
-   * @param {!Object} payload
-   */
-  constructor(payload) {
-    super(() => Promise.resolve(payload.postData || ''));
-    this._response = null;
-    this.url = payload.url;
-    this.method = payload.method;
-    this.headers = new Map(Object.entries(payload.headers));
-  }
-
-  /**
-   * @return {?Response}
-   */
-  response() {
-    return this._response;
-  }
-}
-helper.tracePublicAPI(Request);
-
-class Response extends Body {
-  /**
-   * @param {?Request} request
-   * @param {!Object} payload
-   * @param {function():!Promise<!Buffer>} contentCallback
-   */
-  constructor(request, payload, contentCallback) {
-    super(contentCallback);
-    this._request = request;
-    this.headers = new Map(Object.entries(payload.headers));
-    this.ok = payload.status >= 200 && payload.status <= 299;
-    this.status = payload.status;
-    this.statusText = payload.statusText;
-    this.url = payload.url;
-  }
 
   /**
    * @return {?Response}