fix msg precendence when sending a merging object with msg and err (#1654)

Author: leon-volq

lib/proto.js

@@ -193,7 +193,7 @@ function write (_obj, msg, num) {
     }
   } else {
     obj = _obj
-    if (msg === undefined && _obj[errorKey]) {
+    if (msg === undefined && _obj.msg === undefined && _obj[errorKey]) {
       msg = _obj[errorKey].message
     }
   }

test/error.test.js

@@ -356,3 +356,23 @@ test('correctly adds msg on error when nestedKey is used', async ({ same }) => {
     msg: 'msg message'
   })
 })
+
+test('msg should take precedence over error message on mergingObject', async ({ same }) => {
+  const err = new Error('myerror')
+  const stream = sink()
+  const instance = pino(stream)
+  instance.error({ msg: 'my message', err })
+  const result = await once(stream, 'data')
+  delete result.time
+  same(result, {
+    pid,
+    hostname,
+    level: 50,
+    err: {
+      type: 'Error',
+      stack: err.stack,
+      message: err.message
+    },
+    msg: 'my message'
+  })
+})