Add Headers#keys iterator

fetch.js

@@ -77,6 +77,17 @@
     }, this)
   }
 
+  Headers.prototype.keys = function() {
+    var items = []
+    this.forEach(function(value, name) { items.push(name) })
+    return {
+      next: function() {
+        var value = items.shift()
+        return {done: value === undefined, value: value}
+      }
+    }
+  }
+
   function consumed(body) {
     if (body.bodyUsed) {
       return Promise.reject(new TypeError('Already read'))

test/test.js

@@ -220,6 +220,18 @@ suite('Headers', function() {
       assert.equal(this, thisArg)
     }, thisArg)
   })
+  test('is iterable with keys', function() {
+    var headers = new Headers()
+    headers.append('Accept', 'application/json')
+    headers.append('Accept', 'text/plain')
+    headers.append('Content-Type', 'text/html')
+
+    var iterator = headers.keys()
+    assert.deepEqual({done: false, value: 'accept'}, iterator.next())
+    assert.deepEqual({done: false, value: 'accept'}, iterator.next())
+    assert.deepEqual({done: false, value: 'content-type'}, iterator.next())
+    assert.deepEqual({done: true, value: undefined}, iterator.next())
+  })
  })
 
 // https://fetch.spec.whatwg.org/#request-class