Presently, there is no way to test this, since mochitest runs all of its traffic through a fake web-proxy. I have tested by hand, reproduced the problem, and verified the fix works.
Bug 1702417 Comment 34 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Presently, there is no way to test this in an automated fashion, since mochitest runs all of its traffic through a fake web-proxy. I have tested by hand, reproduced the problem, and verified the fix works.