stream: avoid instanceof

PR-URL: https://github.com/nodejs/node/pull/10558
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

lib/_stream_writable.js

@@ -284,9 +284,11 @@ function decodeChunk(state, chunk, encoding) {
 // If we return false, then we need a drain event, so set that flag.
 function writeOrBuffer(stream, state, isBuf, chunk, encoding, cb) {
   if (!isBuf) {
-    chunk = decodeChunk(state, chunk, encoding);
+    var newChunk = decodeChunk(state, chunk, encoding);
-    if (chunk instanceof Buffer)
+    if (chunk !== newChunk) {
       encoding = 'buffer';
+      chunk = newChunk;
+    }
   }
   var len = state.objectMode ? 1 : chunk.length;