Fix SOAP: Impossible to attach tags to issues

Caused by assigning to unused variable $p_user_id $p_user_id === NULL.
Not sure why this has not come up before, as this bug was actually
introduced a long time ago (a602c5edb5335c46a506166ef68218c7bf3e50de).

Fixes #16484
Tests: Move dateToUTC() to SoapBase class

This helper function can be used everywhere we compare dates, not just
in VersionTest class.

Issues #15817, #16485
Fix #16485: SOAP API test failure for due date