Description:
------------
After doing something like 
$oldsessiondata = $_SESSION;
the changes to $_SESSION won't be committed to the session itself anymore.

I don't think $_SESSION should lose its magic properties in this case.

Reproduce code:
---------------
<?php
session_start();
$_SESSION['testbefore'] = 'foobefore';
print_r($_SESSION);
$oldsessiondata = $_SESSION;
print_r($_SESSION);
$_SESSION['test'] = 'foo';
print_r($_SESSION);
?>

Expected result:
----------------
Array
(
    [testbefore] => foobefore
)
Array
(
    [testbefore] => foobefore
)
Array
(
    [testbefore] => foobefore
    [test] => foo
)

After reload:
Array
(
    [testbefore] => foobefore
    [test] => foo
)
Array
(
    [testbefore] => foobefore
    [test] => foo
)
Array
(
    [testbefore] => foobefore
    [test] => foo
)

Actual result:
--------------
Array
(
    [testbefore] => foobefore
)
Array
(
    [testbefore] => foobefore
)
Array
(
    [testbefore] => foobefore
    [test] => foo
)

After reload:
Array
(
    [testbefore] => foobefore
)
Array
(
    [testbefore] => foobefore
)
Array
(
    [testbefore] => foobefore
    [test] => foo
)