fix for fs.readFile to return string when encoding specified on zero length read

lib/fs.js

@@ -56,7 +56,7 @@ fs.readFile = function (path, encoding_, callback) {
       function doRead() {
         if (size < 1) {
           binding.close(fd);
-          callback(null, buffer);
+          callback(null, encoding ? '' : buffer);
           return;
         }
         // position is offset or null so we can read files on unseekable mediums

test/simple/test-fs-readfile-empty.js

@@ -9,3 +9,7 @@ var
 fs.readFile(fn, function(err, data) {
   assert.ok(data);
 });
+
+fs.readFile(fn, 'utf8', function(err, data) {
+  assert.strictEqual('', data);
+});