feat: Consistent header support for client-side

lib/fetch.browser.js

@@ -156,9 +156,23 @@ function verifyAndExtendResponse(options, response) {
            (max === false || code <= max);
   }
 
+  var originalHeaders = response.headers;
+  var cachedHeaders;
+  function getCachedHeaders() {
+    if (!cachedHeaders) {
+      cachedHeaders = {};
+      originalHeaders.forEach(function addHeader(value, name) {
+        if (name) {
+          cachedHeaders[name] = value;
+        }
+      });
+    }
+    return cachedHeaders;
+  }
+
   Object.defineProperties(response, {
     statusCode: { value: response.status },
-    headers: { value: {} },
+    headers: { get: getCachedHeaders },
   });
 
   function generateStatusCodeError(code) {

test/fetch-http.test.js

@@ -81,6 +81,10 @@ describe('fetch: the basics', function () {
   });
 
   it('exposes a promise to a response body stream', function () {
+    if (typeof document !== 'undefined') {
+      // Streams in the browser and streams in node are two different things.
+      return this.skip();
+    }
     function concat(stream) {
       return new Bluebird(function (resolve, reject) {
         stream.on('error', reject);
@@ -97,4 +101,13 @@ describe('fetch: the basics', function () {
         assert.equal('ok', '' + body);
       });
   });
+
+  it('returns response headers', function () {
+    // this is a silly test in node but is relevant to browser usage
+    return fetch('/test/path', options)
+      .then(function (res) {
+        // Testing content-language b/c it's a "simple response header"
+        assert.equal('has%20stuff', res.headers['content-language']);
+      });
+  });
 });

test/mock-service.js

@@ -23,6 +23,9 @@ function send404(req, res) {
 }
 
 function handleRequest(req, res) {
+  // A random header that happens to be a "simple response header".
+  // See: https://www.w3.org/TR/cors/#simple-response-header
+  res.setHeader('Content-Language', 'has%20stuff');
   if (req.headers.origin) {
     res.setHeader('Access-Control-Allow-Origin', req.headers.origin);
   }