Add support for Response.json static method

fetch.js

@@ -499,6 +499,19 @@ Response.error = function() {
   return response
 }
 
+Response.json = function(data, options) {
+  var responseInit = options ? {
+    headers: new Headers(options.headers),
+    status: options.status,
+    statusText: options.statusText
+  } : {
+    headers: new Headers()
+  }
+  responseInit.headers.set("content-type", "application/json")
+
+  return new Response(JSON.stringify(data), responseInit)
+}
+
 var redirectStatuses = [301, 302, 303, 307, 308]
 
 Response.redirect = function(url, status) {

test/test.js

@@ -702,6 +702,26 @@ exercise.forEach(function(exerciseMode) {
         assert.equal(r.type, 'error')
       })
 
+      test('json creates json Response with ResponseInit', function() {
+        var r = Response.json({key: "value"}, {
+          headers: new Headers('header', 'value'),
+          status: 404,
+          statusText: "NOT FOUND"
+        })
+        assert(r instanceof Response)
+        assert.equal(r.headers.get('content-type'), 'application/json')
+        assert.equal(r.status, 404)
+        assert.equal(r.statusText, "NOT FOUND")
+      })
+
+      test('json creates json Response without ResponseInit', function() {
+        var r = Response.json({key: "value"})
+        assert(r instanceof Response)
+        assert.equal(r.headers.get('content-type'), 'application/json')
+        assert.equal(r.status, 200)
+        assert.equal(r.statusText, '')
+      })
+
       test('redirect creates redirect Response', function() {
         var r = Response.redirect('https://fetch.spec.whatwg.org/', 301)
         assert(r instanceof Response)