Fix asJSON defaults option, make it work for repeated fields.

Fixing two issues in here:

1. When the `defaults` option was enabled, we were using `Object.keys(this)`
   which ends up returning an array that includes things like `$root` and
   asJSON. Instead we can use the `fields` we already have available.

2. For repeated values we were not setting the empty list default because the
   length check would break earlier. We're now passing through when the
   defaults option is set as well.

Added a test on this stuff as well.

src/message.js

@@ -3,7 +3,7 @@ module.exports = Message;
 
 /**
  * Constructs a new message instance.
- * 
+ *
  * This method should be called from your custom constructors, i.e. `Message.call(this, properties)`.
  * @classdesc Abstract runtime message.
  * @extends {Object}
@@ -46,17 +46,15 @@ MessagePrototype.asJSON = function asJSON(options) {
         json   = {};
     var keys;
     if (options.defaults) {
-        keys = [];
-        for (var k in this) // eslint-disable-line guard-for-in
-            keys.push(k);
+        keys = Object.keys(fields);
     } else
         keys = Object.keys(this);
     for (var i = 0, key; i < keys.length; ++i) {
         var field = fields[key = keys[i]],
             value = this[key];
         if (field) {
             if (field.repeated) {
-                if (value && value.length) {
+                if (value && (value.length || options.defaults)) {
                     var array = new Array(value.length);
                     for (var j = 0, l = value.length; j < l; ++j)
                         array[j] = field.jsonConvert(value[j], options);

tests/data/message.proto

@@ -0,0 +1,8 @@
+syntax = "proto3";
+
+package test;
+
+message TestRepeatedDefaults {
+  required string aString = 1;
+  repeated string repeatString = 2;
+}

tests/message.js

@@ -0,0 +1,25 @@
+var tape = require("tape");
+var protobuf  = require("..");
+
+tape.test("test asJSON repeated defaults", function(test) {
+
+    protobuf.load("tests/data/message.proto", function(err, root) {
+        if (err)
+            return test.fail(err.message);
+        test.ok(true, "should parse without errors");
+
+        var TestMessage = root.lookup("test.TestRepeatedDefaults");
+        var msg1 = TestMessage.create({ aString: 'foo' });
+
+        var defaultsOn = msg1.asJSON({ defaults: true });
+        test.equal(defaultsOn.aString, 'foo', "should set aString value");
+        test.same(defaultsOn.repeatString, [], "should set repeated default");
+
+        var defaultsOff = msg1.asJSON({ defaults: false });
+        test.equal(defaultsOff.aString, 'foo', "should set aString value");
+        test.same(defaultsOff.repeatString, undefined, "should not set repeated default");
+
+        test.end();
+    });
+
+});