Add fetch's clone function to Request and Response prototypes

Summary: Fetch includes a cloning function that was not included in the React Native fetch.js. This adds it in so that cloning is available on responses from fetch calls in React Native. The code is taken directly from [fetch](https://github.com/github/fetch/blob/master/fetch.js) on Github.

<img width="601" alt="screen shot 2015-10-22 at 11 50 53 am" src="https://cloud.githubusercontent.com/assets/11564650/10675101/304ebe5a-78b3-11e5-9d6b-24ea6d9fb998.png">

<img width="596" alt="screen shot 2015-10-22 at 12 20 08 pm" src="https://cloud.githubusercontent.com/assets/11564650/10675834/4abaf4ee-78b7-11e5-9d34-436290b64b30.png">
Closes #3614

Reviewed By: svcscm

Differential Revision: D2600517

Pulled By: sahrens

fb-gh-sync-id: d70c5c58e923d997f93bb5aa2d1d90e95d5a75f2

Libraries/Fetch/fetch.js

@@ -278,6 +278,10 @@ var self = {};
     }
     this._initBody(body)
   }
+
+  Request.prototype.clone = function() {
+    return new Request(this)
+  }
 
   function decode(body) {
     var form = new FormData()
@@ -320,6 +324,15 @@ var self = {};
     this.headers = options.headers instanceof Headers ? options.headers : new Headers(options.headers)
     this.url = options.url || ''
   }
+
+  Response.prototype.clone = function() {
+    return new Response(this._bodyInit, {
+      status: this.status,
+      statusText: this.statusText,
+      headers: new Headers(this.headers),
+      url: this.url
+    })
+  }
 
   Body.call(Response.prototype)