Description:
------------
A NumberFormatter instance retrieved from the session generates an 

'Found unconstructed NumberFormatter: U_ILLEGAL_ARGUMENT_ERROR'

on calling ::format(). 

The code snippet below can be run from the in-built webserver.  The second time the page is requested (within a session), the above error is reported.

Test script:
---------------
session_start();

if(isset($_SESSION["currencyFormatter"])){
    $formatter = $_SESSION["currencyFormatter"];
    echo "loaded formatter from session\n";
} else {
    echo "no formatter on session\n";
    $formatter = new NumberFormatter("en_GB", NumberFormatter::CURRENCY);
    $_SESSION["currencyFormatter"] = $formatter;
}

var_dump($formatter->format(123, NumberFormatter::CURRENCY));
if(intl_is_failure($formatter->getErrorCode())) {
    echo "\n" . $formatter->getErrorMessage();
}

session_commit();

Expected result:
----------------
First request:

no formatter on session
string '£123.00' (length=8)

Subsequent requests:

loaded formatter from session
string '£123.00' (length=8)

Actual result:
--------------
First request:

no formatter on session
string '£123.00' (length=8)

Subsequent requests:

loaded formatter from session
boolean false
Found unconstructed NumberFormatter: U_ILLEGAL_ARGUMENT_ERROR