FIX: Allow calling sender.close() multiple times

[martino87r]

Fixes an incorrect import in the tutorial section of the documentation

[codecov-commenter]

Codecov Report

Base: 99.66% // Head: 99.66% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (850295f) compared to base (46908b1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   99.66%   99.66%           
=======================================
  Files          10       10           
  Lines         589      590    +1     
=======================================
+ Hits          587      588    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
redmail/email/sender.py	100.00% <100.00%> (ø)

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[Miksus]

Ye, indeed. Thanks for spotting!

I was thinking about the close. Now it wouldn't raise an exception if the connection is already closed. Considering the python file buffer (ie. open("myfile.txt", "w")) does not raise an exception if the buffer is already closed, I think your change is appropriate.