If our connection is explicitly non-strict, tell MySQL

We default to making the connection strict, but have historically relied
on the MySQL default when we want it to be non-strict. On some (recent?)
versions of MySQL, new connections default to being strict, so if we've
been told 'strict:false', we're obliged to pass that on.

This fixes a test failure that we've seen turn up on relatively-new
development machines, so we do already have a test covering it.
Merge pull request #16065 from matthewd/mysql-explicit-non-strict

If our connection is explicitly non-strict, tell MySQL