Support list-unsubscribe header formatting better in the conformance fixer.

[tommairs]

While testing adding List-Unsubscribe headers, I noticed that extra spaces were being introduced in inappropriate places.
This caused the headers to be malformed and so were ignored by the MBPs.

This turned out to be a conflict in how check_fix_conformance works with header line lengths

We need to teach the mail parser about the list-unsubscribe syntax and have the conformance fixer compensate. In the meantime, if we document that the conformance fixer should always come first in the event handler, then this can conflict be avoided.