fix SplFileInfo->openFile

without a default class, opening a file doens't work so well

Closes #802

bin/systemlib.php

@@ -887,7 +887,7 @@ class SplMinHeap extends SplHeap implements Iterator, Countable {
 class SplFileInfo {
 
   private $fileName;
-  private $fileClass;
+  private $fileClass = "SplFileObject";
   private $infoClass;
 
   // This doc comment block generated by idl/sysdoc.php

hphp/system/classes/spl/file_handling/SplFileInfo.php

@@ -11,7 +11,7 @@
 class SplFileInfo {
 
   private $fileName;
-  private $fileClass;
+  private $fileClass = "SplFileObject";
   private $infoClass;
 
   // This doc comment block generated by idl/sysdoc.php

hphp/test/slow/ext_spl_file/openFile.php

@@ -0,0 +1,4 @@
+<?php
+
+$info = new SplFileInfo(__DIR__.'/../../sample_dir/file');
+var_dump($info->openFile()->fgets());

hphp/test/slow/ext_spl_file/openFile.php.expect

@@ -0,0 +1,2 @@
+string(3) "hi
+"