Attachment fixes

[lefta]

This PR is made of three commits.

The first one is pretty straightforward. It sets __version__ to avoid crashing in development mode with python 3.

The second one fixes a crash when uploading an attachment containing non ascii characters in its file name ( #165 ). The problem came from rfc6266.build_header() which returns a bytes object in this case :astonished: , making the python header builder crash. So I just decode it when it is the case.

The third one allows to upload empty attachments. I agree that there is no point in doing this, but as much as I know this is not forbidden in mail transport specifications. And the resulting error, None, is ... not really an error in fact :smile:

Note that I did not test it with python 2, but there should not be any problem, these are small changes.

If these changes are accepted, may I ask for a release soon ? As I said in the bug report, I really need it to be fixed. ( Just asking, feel free to answer no if this bothers you )

[codecov[bot]]

Codecov Report

Merging #166 into master will decrease coverage by 0.1%. The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   74.26%   74.15%   -0.11%     
==========================================
  Files          23       23              
  Lines        2036     2039       +3     
==========================================
  Hits         1512     1512              
- Misses        524      527       +3

Impacted Files	Coverage Δ
modoboa_webmail/__init__.py	75% <0%> (ø)	:arrow_up:
modoboa_webmail/lib/attachments.py	62.68% <0%> (-2.94%)	:arrow_down:
modoboa_webmail/forms.py	76.6% <100%> (ø)	:arrow_up:

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[tonioo]

@lefta Thank you.