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(