[bug?][notmuch-emacs] X11: renders Headers with face attributes

[Gregor Zattler]

Dear notmuch developers,

today I got a spam mail which's Subject: and From: header were
displayed as bold (with the exception of German umlauts) in
notmuch-emacs in a graphical frame (X11), they were displayed as
a string of hollow rectangles in a frame on a urxvt terminal (see
attached screen shots).  I see there are bold and italic
mathematical bold letters in Unicode.  The bold ones are used
here.

This are the headers in question:

Subject:=?UTF-8?B?IPCdl5jwnZe78J2YgfCdl7nwnZeu8J2YgPCdmIHwnZiC8J2Xu/Cdl7Qg8J2Xs8O88J2XvyDwnZeU8J2Xu/Cdl7TwnZey8J2XtcO28J2Xv/Cdl7bwnZe08J2XsiDwnZe28J2XuiDwnZen8J2Xv/Cdl67wnZiC8J2XsvCdl7/wnZez8J2XrvCdl7nwnZe5IA==?=
From:=?UTF-8?B?IPCdl6bwnZiB8J2XsvCdl7/wnZev8J2Xsi3wnZep8J2XvPCdl7/wnZiA8J2XvPCdl7/wnZe08J2XsiA=?=<2mH2r at 0ytix511n5cb63oezbgj69wpm.com>

Is this a bug?  Should untrusted senders be allowed to influence
how headers are displayed?  I don't think so.  For me this case
should be handled as a homograph attack
(https://en.wikipedia.org/wiki/IDN_homograph_attack).

I do know there is character folding for searches but emacs would
need to use the reverse for displaying characters and only
characters which deviate in a way that is recognized as a font
attribute.

Thanks for your attention, Gregor
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