Inconsistent handling of space after subject prefix

A space char is added between subject prefix and original subject, but only when subject is not MIME-encoded. It should be added on mime-encoded subjects, too (alternatively nothing should be added in both cases).

Example:

configured prefix is "[test]"

original Subject: plain
munged Subject: [test] plain

-> with space

original Subject: =?utf-8?q?encoded?=
munged Subject: =?utf-8?q?=5Btest=5Dencoded?=

-> without space

Mailman 3.3.3 (Debian Bullseye)