Service Workers/Fetch: Fix Response constructor for empty/null

LayoutTests/http/tests/serviceworker/resources/response-content-worker.js

 importScripts('worker-testharness.js');
 importScripts('../../resources/testharness-helpers.js');
 
 promise_test(function() {
     var response = new Response('test string');
     assert_equals(
       response.headers.get('Content-Type'),
       'text/plain;charset=UTF-8',
       'A Response constructed with a string should have a Content-Type.');
     return response.text()
@@ skipping 83 matching lines @@
           response3 = response.clone();
           assert_true(response3.bodyUsed,
                       'bodyUsed should be true in clone of consumed response.');
           return response2.text();
         })
       .then(function(text) {
           assert_equals(text, 'test string',
                         'Response clone response body text should match.');
         });
   }, 'Behavior of bodyUsed in Response and clone behavior.');
+
+promise_test(function() {
+    var response = new Response(null);
+    assert_equals(
+      response.headers.get('Content-Type'),
+      'text/plain;charset=UTF-8',
+      'A Response constructed with a value coerced to string should have a ' +
+      'text Content-Type.');
+    return response.text()
+      .then(function(text) {
+          assert_equals(text, 'null',
+                        'A null value passed to Response constructor should ' +
+                        'be coerced to the string "null".');
+        });
+  }, 'Behavior of Response passed null for body.');
+
+promise_test(function() {
+    var response = new Response();
+    assert_equals(
+      response.headers.get('Content-Type'),
+      null,
+      'A Response constructed with no body should have no Content-Type.');
+    return response.text()
+      .then(function(text) {
+          assert_equals(text, '',
+                        'Response with no body accessed as text should ' +
+                        'resolve to the empty string.');
+        });
+  }, 'Behavior of Response with no body.');