promises: use encoding in readfile

Use the encoding parameter passed to fsPromises.readFile if it is
passed. Currently the encoding parameter is ignored in fsPromises

PR-URL: nodejs#19296
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Fixes: nodejs#19286

lib/fs/promises.js

@@ -157,7 +157,12 @@ async function readFileHandle(filehandle, options) {
       chunks.push(buffer.slice(0, totalRead));
   } while (totalRead === chunkSize);
 
-  return Buffer.concat(chunks);
+  var buffer = Buffer.concat(chunks);
+  if (options.encoding) {
+    return buffer.toString(options.encoding);
+  } else {
+    return buffer;
+  }
 }
 
 // All of the functions are defined as async in order to ensure that errors

test/parallel/test-fs-promises-writefile.js

@@ -35,7 +35,15 @@ async function doRead() {
   assert.deepStrictEqual(buf, data);
 }
 
+async function doReadWithEncoding() {
+  const data = await fsPromises.readFile(dest, 'utf-8');
+  const syncData = fs.readFileSync(dest, 'utf-8');
+  assert.strictEqual(typeof data, 'string');
+  assert.deepStrictEqual(data, syncData);
+}
+
 doWrite()
   .then(doAppend)
   .then(doRead)
+  .then(doReadWithEncoding)
   .then(common.mustCall());