Add Headers.forEach method.

lib/headers.js

@@ -69,6 +69,20 @@ Headers.prototype.getAll = function(name) {
 	return this._headers[name.toLowerCase()];
 };
 
+/**
+ * Iterate over all headers
+ * @param  Function callback Executed for each item with parameters (value, name, thisArg)
+ * @param  Boolean  thisArg  `this` context for callback function
+ * @return Void
+ */
+Headers.prototype.forEach = function(callback, thisArg) {
+	Object.getOwnPropertyNames(this._headers).forEach(function(name) {
+		this._headers[name].forEach(function(value) {
+	    	callback.call(thisArg, value, name, this)
+	  	}, this)
+	}, this)
+}
+
 /**
  * Overwrite header values given name
  *

test/test.js

@@ -735,6 +735,29 @@ describe('node-fetch', function() {
 		});
 	});
 
+	it('should allow iterating through all headers', function() {
+		var headers = new Headers({
+			a: 1,
+			b: [2, 3],
+		});
+
+		var myHeaders = [];
+		function callback(value, name) {
+			myHeaders.push([name, value]);
+		}
+
+		expected = [
+			["a", "1"],
+			["b", "2"],
+			["b", "3"]
+		];
+
+		expect(headers.forEach).to.be.defined;
+
+		headers.forEach(callback, headers);
+		expect(myHeaders).to.be.deep.equal(expected);
+	});
+
 	it('should allow deleting header', function() {
 		url = base + '/cookie';
 		return fetch(url).then(function(res) {