Release/0.8.0: When unbinding and disconnecting from SMSC, set SMPP session state to CLOSED

[komuw]

Thank you for contributing to naz.
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Contributions are under the MIT License.

Answer the following questions,

What(What have you changed?)

• When unbinding and disconnecting from SMSC, set SMPP session state to CLOSED
• add a test to check that; during shutdown we close the writer & not reader

Why(Why did you change it?)

• Updates: https://github.com/komuw/naz/issues/211
• Updates: https://github.com/komuw/naz/issues/197

References:

[codecov-commenter]

Codecov Report

Merging #213 into master will increase coverage by 0.23%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   84.59%   84.82%   +0.23%     
==========================================
  Files          15       15              
  Lines        1564     1568       +4     
==========================================
+ Hits         1323     1330       +7     
+ Misses        241      238       -3     

Impacted Files	Coverage Δ
naz/client.py	81.53% <100.00%> (+0.56%)	:arrow_up:

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[komuw]

superseded by https://github.com/komuw/naz/pull/214