encode date is null fix (#85)

* date is null fix

* fix format

* unit test for date null

* unit test for date null

* remove comment in code

* add use strict

* fix format

Author: zheli-1

lib/codecs/DateCodec.js

@@ -1,6 +1,10 @@
 const type = -1
 
 function encode (dt) {
+  if (dt === null) {
+    return
+  }
+
   var millis = dt * 1
   var seconds = Math.floor(millis / 1000)
   var nanos = (millis - seconds * 1000) * 1e6

test/datenull.js

@@ -0,0 +1,13 @@
+'use strict'
+var test = require('tape').test
+var msgpack = require('../')
+
+test('encode date is null ', function (t) {
+  var encoder = msgpack({
+    disableTimestampEncoding: true
+  })
+
+  t.equal(encoder.encode(null)[0], 0xc0, 'encode null as null')
+
+  t.end()
+})