Unicode headers

[awdeorio]

Test messages with unicode characters in TO, FROM and SUBJECT. This exposed a bunch of bugs that led to fixing #80 as well.

Closes #49, closes #80

This PR contains a work-around for a bug in future.backports.email.base64mime.header_encode that raises an exception when serializing an email message object containing a base64-encoded UTF8 header. PythonCharmers/python-future#555

[codecov-io]

Codecov Report

Merging #88 into develop will decrease coverage by 0.62%.
The diff coverage is 92.85%.

@@             Coverage Diff             @@
##           develop      #88      +/-   ##
===========================================
- Coverage    99.65%   99.02%   -0.63%     
===========================================
  Files            5        6       +1     
  Lines          287      309      +22     
===========================================
+ Hits           286      306      +20     
- Misses           1        3       +2     

Impacted Files	Coverage Δ
mailmerge/template_message.py	100.00% <ø> (ø)
mailmerge/utils.py	90.00% <90.00%> (ø)
mailmerge/__main__.py	99.21% <100.00%> (+<0.01%)	⬆️
mailmerge/sendmail_client.py	100.00% <100.00%> (ø)

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