stream: Pass 'buffer' encoding to decoded writables

Since the encoding is no longer relevant once it is decoded to a Buffer,
it is confusing and incorrect to pass the encoding as 'utf8' or whatever
in those cases.

Closes nodejs#6119

lib/_stream_writable.js

@@ -196,6 +196,8 @@ function decodeChunk(state, chunk, encoding) {
 // If we return false, then we need a drain event, so set that flag.
 function writeOrBuffer(stream, state, chunk, encoding, cb) {
   chunk = decodeChunk(state, chunk, encoding);
+  if (Buffer.isBuffer(chunk))
+    encoding = 'buffer';
   var len = state.objectMode ? 1 : chunk.length;
 
   state.length += len;

test/simple/test-stream-writable-decoded-encoding.js

@@ -0,0 +1,60 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+var stream = require('stream');
+var util = require('util');
+
+function MyWritable(fn, options) {
+  stream.Writable.call(this, options);
+  this.fn = fn;
+};
+
+util.inherits(MyWritable, stream.Writable);
+
+MyWritable.prototype._write = function (chunk, encoding, callback) {
+  this.fn(Buffer.isBuffer(chunk), typeof chunk, encoding);
+  callback();
+};
+
+;(function decodeStringsTrue() {
+  var m = new MyWritable(function(isBuffer, type, enc) {
+    assert(isBuffer);
+    assert.equal(type, 'object');
+    assert.equal(enc, 'buffer');
+    console.log('ok - decoded string is decoded');
+  }, { decodeStrings: true });
+  m.write('some-text', 'utf8');
+  m.end();
+})();
+
+;(function decodeStringsFalse() {
+  var m = new MyWritable(function(isBuffer, type, enc) {
+    assert(!isBuffer);
+    assert.equal(type, 'string');
+    assert.equal(enc, 'utf8');
+    console.log('ok - un-decoded string is not decoded');
+  }, { decodeStrings: false });
+  m.write('some-text', 'utf8');
+  m.end();
+})();