return a six.text_type from _deserialize_string if None is passed

[codecov[bot]]

Codecov Report

Merging #111 (04e0b11) into master (c704874) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #111   +/-   ##
=======================================
  Coverage   98.42%   98.42%           
=======================================
  Files          13       13           
  Lines        3296     3296           
=======================================
  Hits         3244     3244           
  Misses         52       52           

Impacted Files	Coverage Δ
pypsrp/client.py	97.58% <100.00%> (ø)
pypsrp/serializer.py	100.00% <100.00%> (ø)

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[jborean93]

Thanks for the pickup.