Slack notification failures with some proxies

In the past few days we have noted that our Slack notifications have stopped. After some experimentation with both './test_alert.php' (which failed) and the curl debug output it produces (which worked) I've narrowed this down to the PHP stream context option 'request_fulluri'.

I assume that one of our proxy servers has been updated, causing this issue to appear, but if I now remove the setting of 'request_fulluri' in 'includes/common.inc.php' (setting it back to its default) then './test_alert.php' works correctly.

Could the setting of 'request_fulluri' be added as a configuration option so that we can resolve this without having to edit the code?