Serialize errors with cause (pinojs#105)

* Add (failing) test for serialization of causes of errors

* Add cause serialization to error serializer

The error serializer already handles the case where an error is not of type Error (or AggregateError) well, so there is no reason to add special handling for that. We can simply pass whatever is in `cause` into the serializer, and we should get a serialized result out.

* Add `cause` to errors in similar way to what is done in other errors

* Expand cause serialization test to show recursion

lib/err.js

@@ -28,6 +28,11 @@ const pinoErrProto = Object.create({}, {
     writable: true,
     value: undefined
   },
+  cause: {
+    enumerable: true,
+    writable: true,
+    value: undefined
+  },
   raw: {
     enumerable: false,
     get: function () {
@@ -60,6 +65,10 @@ function errSerializer (err) {
     _err.aggregateErrors = err.errors.map(err => errSerializer(err))
   }
 
+  if (err.cause) {
+    _err.cause = errSerializer(err.cause)
+  }
+
   for (const key in err) {
     if (_err[key] === undefined) {
       const val = err[key]

test/err.test.js

@@ -194,3 +194,27 @@ test('serializes aggregate errors', { skip: !global.AggregateError }, function (
   t.match(serialized.aggregateErrors[1].stack, /^Error: bar/)
   t.match(serialized.stack, /err\.test\.js:/)
 })
+
+test('serializes causes', function (t) {
+  t.plan(11)
+
+  const bar = new Error('bar')
+  bar.cause = new Error('foo')
+  bar.cause.cause = new Error('baz')
+
+  const serialized = serializer(bar)
+
+  t.equal(serialized.type, 'Error')
+  t.equal(serialized.message, 'bar: foo: baz') // message serialization already walks cause-chain
+  t.match(serialized.stack, /err\.test\.js:/)
+
+  t.ok(serialized.cause)
+  t.equal(serialized.cause.type, 'Error')
+  t.equal(serialized.cause.message, 'foo: baz')
+  t.match(serialized.cause.stack, /err\.test\.js:/)
+
+  t.ok(serialized.cause.cause)
+  t.equal(serialized.cause.cause.type, 'Error')
+  t.equal(serialized.cause.cause.message, 'baz')
+  t.match(serialized.cause.cause.stack, /err\.test\.js:/)
+})