Description:
------------
event_buffer_enable returns false when I use fopen. But it 
works normal when I use popen() or fsockopen().

Reproduce code:
---------------
<?php
function print_line($buf, $arg) {echo event_buffer_read($buf, 4096);}
function error_func($buf, $what, $arg) {}
$base = event_base_new();
$fp = fopen(__FILE__,'r');
$eb = event_buffer_new($fp, "print_line", NULL, "error_func", $base);
$set = event_buffer_base_set($eb, $base);
$enabled = event_buffer_enable($eb, EV_READ);
var_dump(array(
 'fp' => $fp,
 'eb' => $eb,
 'set' => $set,
 'enabled' => $enabled)
);
event_base_loop($base);


Expected result:
----------------
array(4) {
  ["fp"]=>
  resource(6) of type (stream)
  ["eb"]=>
  resource(7) of type (buffer event)
  ["set"]=>
  bool(true)
  ["enabled"]=>
  bool(true)
}
// source code bellow


Actual result:
--------------
array(4) {
  ["fp"]=>
  resource(6) of type (stream)
  ["eb"]=>
  resource(7) of type (buffer event)
  ["set"]=>
  bool(true)
  ["enabled"]=>
  bool(false)
}