wddx_serialize_value() does not escape "&"s

[scfc]

With PHP 5.5.18, the test script:

<?php
$s = wddx_serialize_value("Test for &");
var_dump($s);
$d = wddx_deserialize($s);
var_dump($d);

gives:

string(92) "<wddxPacket version='1.0'><header/><data><string>Test for &amp;</string></data></wddxPacket>"
string(10) "Test for &"

HipHop VM 3.3.1 fails with:

string(88) "<wddxPacket version='1.0'><header/><data><string>Test for &</string></data></wddxPacket>"
Entity: line 1: parser error : xmlParseEntityRef: no name
<wddxPacket version='1.0'><header/><data><string>Test for &</string></data></wdd
                                                           ^
NULL

The "&" needs to be escaped for the resulting WDDX to be valid XML.

[scfc]

(Downstream WMF bug: https://bugzilla.wikimedia.org/73531.)

[paulbiss]

Fix is up internally D1691695