Description:
------------
If there are some outputs on the functions called by `register_shutdown_function`, the `ob_gzhandler` do not work correctly for these outputs messages. In other words, these messages have not been compressed with the other outpus msg.

It is serious problem for the Web Browsers, because they will get the error check sum of contents, so the page content can't not be display fullly.

Reproduce code:
---------------
<?php
function foo()
{
  echo "Hello, foo bar before process quit!\n";
}

ob_start('ob_gzhandler');
register_shutdown_function('foo');
echo "Normal output\n";
?>

Expected result:
----------------
The output of function `foo` will be added to all output msg and compressed by ob_gzhandler

Actual result:
--------------
"Normal output\n" were compressed, and the ouput of `foo()` never been compressed.