Do not cast to string Error#stack. (#464)

pino.js

@@ -147,7 +147,9 @@ function asJson (obj, msg, num, time) {
     var notHasOwnProperty = obj.hasOwnProperty === undefined
     if (objError) {
       data += ',"type":"Error"'
-      data += obj.stack ? ',"stack":' + this.stringify(obj.stack) : ''
+      if (obj.stack !== undefined) {
+        data += ',"stack":' + this.stringify(obj.stack)
+      }
     }
     // if global serializer is set, call it first
     if (this.serializers[Symbol.for('pino.*')]) {

test/error.test.js

@@ -143,3 +143,19 @@ test('stack is omitted if it is not set on err', t => {
   instance.level = name
   instance[name](err)
 })
+
+test('stack is rendered as any other property if it\'s not a string', t => {
+  t.plan(3)
+  var err = new Error('myerror')
+  err.stack = null
+  var instance = pino(sink(function (chunk, enc, cb) {
+    t.ok(new Date(chunk.time) <= new Date(), 'time is greater than Date.now()')
+    delete chunk.time
+    t.equal(chunk.hasOwnProperty('stack'), true)
+    t.equal(chunk.stack, null)
+    cb()
+  }))
+
+  instance.level = name
+  instance[name](err)
+})