support falsy arguments in Object.assign polyfill (#2433)

* support falsy arguments in Object.assign polyfill

* add tests for assign polyfill

pathname/assign.js

@@ -1,5 +1,5 @@
 "use strict"
 
 module.exports = Object.assign || function(target, source) {
-	Object.keys(source).forEach(function(key) { target[key] = source[key] })
+	if(source) Object.keys(source).forEach(function(key) { target[key] = source[key] })
 }

pathname/tests/index.html

@@ -13,6 +13,7 @@
 		<script src="test-buildPathname.js"></script>
 		<script src="test-parsePathname.js"></script>
 		<script src="test-parseTemplate.js"></script>
+		<script src="test-assign.js"></script>
 
 		<script>require("../../ospec/ospec").run()</script>
 	</body>

pathname/tests/test-assign.js

@@ -0,0 +1,26 @@
+"use strict"
+
+var o = require("../../ospec/ospec")
+
+// force usage of polyfill
+var save = Object.assign
+Object.assign = null
+delete require.cache[require.resolve("../assign")]
+var assign = require("../assign")
+Object.assign = save
+
+o.spec("assign polyfill", function() {
+	o("works", function() {
+		var target = {hello: "world", foo: "bar"}
+		var source = {foo: "foo", extra: true}
+
+		assign(target, source)
+
+		o(target).deepEquals({hello: "world", foo: "foo", extra: true})
+
+		var falsySources = [null, 0, "", false, void 0]
+		falsySources.forEach(function(falsy) { assign(target, falsy) })
+
+		o(target).deepEquals({hello: "world", foo: "foo", extra: true})
+	})
+})