JS: ensure that extension values are serialized even if they're falsy

There was a bug where for JavaScript we would only serialize an
extension value if it evaluated as truthy, which meant that values like
0 would get silently dropped (even in proto2, where field presence is
significant). This fixes issue #2605, and takes care of the output of
toObject() in addition to the binary format.

js/binary/proto_test.js

diff --git a/js/binary/proto_test.js b/js/binary/proto_test.js
index f86dc64..f5e1b6b 100644
--- a/js/binary/proto_test.js
+++ b/js/binary/proto_test.js
@@ -283,8 +283,7 @@
  * @param {!proto.jspb.test.TestExtendable} msg
  */
 function checkExtensions(msg) {
-  assertEquals(-42,
-      msg.getExtension(proto.jspb.test.extendOptionalInt32));
+  assertEquals(0, msg.getExtension(proto.jspb.test.extendOptionalInt32));
   assertEquals(-0x7fffffff00000000,
       msg.getExtension(proto.jspb.test.extendOptionalInt64));
   assertEquals(0x80000000,
@@ -512,8 +511,7 @@
    * @param {proto.jspb.test.TestExtendable} msg
    */
   function fillExtensions(msg) {
-    msg.setExtension(
-        proto.jspb.test.extendOptionalInt32, -42);
+    msg.setExtension(proto.jspb.test.extendOptionalInt32, 0);
     msg.setExtension(
         proto.jspb.test.extendOptionalInt64, -0x7fffffff00000000);
     msg.setExtension(

js/message.js

diff --git a/js/message.js b/js/message.js
index 05d34e9..4e2517d 100644
--- a/js/message.js
+++ b/js/message.js
@@ -497,7 +497,7 @@
   for (var fieldNumber in extensions) {
     var fieldInfo = extensions[fieldNumber];
     var value = getExtensionFn.call(proto, fieldInfo);
-    if (value) {
+    if (goog.isDefAndNotNull(value)) {
       for (var name in fieldInfo.fieldName) {
         if (fieldInfo.fieldName.hasOwnProperty(name)) {
           break; // the compiled field name
@@ -541,7 +541,7 @@
                       'without binary serialization support');
     }
     var value = getExtensionFn.call(proto, fieldInfo);
-    if (value) {
+    if (goog.isDefAndNotNull(value)) {
       if (fieldInfo.isMessageType()) {
         // If the message type of the extension was generated without binary
         // support, there may not be a binary message serializer function, and

js/message_test.js

diff --git a/js/message_test.js b/js/message_test.js
index a2c5763..2298742 100644
--- a/js/message_test.js
+++ b/js/message_test.js
@@ -693,10 +693,11 @@
      });
 
   it('testToObject_hasExtensionField', function() {
-    var data = new proto.jspb.test.HasExtensions(['str1', {100: ['ext1']}]);
+    var data = new proto.jspb.test.HasExtensions(['str1', {100: ['ext1'], 102: ''}]);
     var obj = data.toObject();
     assertEquals('str1', obj.str1);
     assertEquals('ext1', obj.extField.ext1);
+    assertEquals('', obj.str);
   });
 
   it('testGetExtension', function() {