Fix workarounds for MUAs that don't handle UTF-8

Turns out that clean_header() takes Header, so we were getting:
  AttributeError
  'Header' object has no attribute 'startswith'

Let's use Header in the test and fix the issue properly.

Fix: a7046dca ("Add workaround for MUAs that don't handle UTF-8")
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
1 job for master in 1 minute and 41 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #3460030
test

00:01:41