Avoid double continuation resume in URLSession delegate in corner case

[simonjbeaumont]

Motivation

If the request completes with error after the response has already been received we can potentially resume a continuation twice. This was shown by running some of the tests in long loop.

Modifications

• Add a shorter test that more reliably reproduced the issue.
• Add the fix: to set the continuation to nil after resuming it, while still holding the lock.

Result

• Removed a source of a potential runtime crash.

Test Plan

• The newly added test, which reliably failed, now reliably passes.