Open requested to merge abompard/mailman:subject-prefix into master
subject_prefix handler has many bugs and was mostly not covered for lists with a non-ascii charset.
This MR fixes most of the cases, but if you ask me this module needs some refactoring. For example, all tests still pass when only the
mixed_charsets function is used.
We can use this MR to fix the current bugs or go with the refactoring (I have it ready).