REGTESTS: converters: use connection: close in the has_ctl() test

REGTESTS: converters: use connection: close in the has_ctl() test

The reg-test used to test has_ctl() introduced by commit 33374957db
("MINOR: sample: add new converter has_ctl() to detect control characters")
is flaky due to the server not using "connection: close", so under load
it randomly shows "HTTP header incomplete". Let's add the close like in
many other similar tests involving server reuse.

This should be backported wherever the converter above is backported.