Cover subject prefix cases for empty subjects and non-ascii charsets

Neal Gompa (ニール・ゴンパ)requested to merge
Conan_Kudo/mailman:subject-prefix into master

The subject_prefix handler has several bugs around empty subjects and lacked coverage for lists with a non-ascii charset.

Obsoletes !70 (closed)

Edited by Neal Gompa (ニール・ゴンパ)

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