Can't view mail-content if quarantined mail is "bad mime"; decoding error

[wlma]

Hi all, i've got some Error-messages in Modoboa-amavis (Version 1.2.2) (Quarantine-View)
Logtrace:
`

Request Method:	GET
https:///quarantine/getmailcontent/JZoZ4BDn1hM0/
1.11.10
DjangoUnicodeDecodeError
'ascii' codec can't decode byte 0xc3 in position 91: ordinal not in range(128). You passed in 'Kommentar zu Ticket: [TT]:28834 [ELVIE38_EIlog_20180312 und ELVIE56_EIlog_20180312 unvollst\xc3\xa4ndig] [ELBA-internet]' (<type 'str'>)
/srv/modoboa/env/local/lib/python2.7/site-packages/django/utils/encoding.py in force_text, line 88
/usr/bin/uwsgi-core
2.7.13
['.', '', '/srv/modoboa/env/lib/python2.7', '/srv/modoboa/env/lib/python2.7/plat-x86_64-linux-gnu', '/srv/modoboa/env/lib/python2.7/lib-tk', '/srv/modoboa/env/lib/python2.7/lib-old', '/srv/modoboa/env/lib/python2.7/lib-dynload', '/usr/lib/python2.7', '/usr/lib/python2.7/plat-x86_64-linux-gnu', '/usr/lib/python2.7/lib-tk', '/srv/modoboa/env/local/lib/python2.7/site-packages', '/srv/modoboa/env/lib/python2.7/site-packages']
`
Subject line includes some non-ascii chars... I'm not sure whether this is allowed by RFC or if the sending Server/myServer is doing it wrong, but maybe it would be nice to catch this somehow.

Regards,
Andreas