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First proposed in #10998.

This patch doesn't quite fix the 'run in a VM with Samba share'
timing problem, but it should at least make it better.

Why I consider this a bug rather than an API change: the API change was
to Message, which didn't used to return Headers unless you added them
yourself.  Now it does (for 8bit binary header input), so decode_header
needs to be able to handle them.

Algorithm and initial patch by Michael Henry.

Patch by Michael Henry.

The fix was already committed to 3.2, but I merged two small changes
recommended by Raymond while I was working on the 2.7 patch to ease
future merges.