Description:
------------
The output_add_rewrite_var can only be used once, the second time it fails.

More on this problem here: http://stackoverflow.com/questions/5822936/output-add-
rewrite-var-problems

Test script:
---------------
// outer buffer
ob_start();

print_r(ob_list_handlers());

// inner buffer 1
ob_start();
output_add_rewrite_var('var', 'value');
print_r(ob_list_handlers());
ob_end_flush();

print_r(ob_list_handlers());

// inner buffer 2
ob_start();
output_add_rewrite_var('var', 'value');
print_r(ob_list_handlers());
ob_end_flush();

print_r(ob_list_handlers());

Expected result:
----------------
Output should have been:

Array
(
    [0] => default output handler
)
Array
(
    [0] => default output handler
    [1] => URL-Rewriter
)
Array
(
    [0] => default output handler
)
Array
(
    [0] => default output handler
    [1] => URL-Rewriter
)
Array
(
    [0] => default output handler
)

Actual result:
--------------
Output is:

Array
(
    [0] => default output handler
)
Array
(
    [0] => default output handler
    [1] => URL-Rewriter
)
Array
(
    [0] => default output handler
)
Array
(
    [0] => default output handler
    [1] => default output handler
)
Array
(
    [0] => default output handler
)