Fix microseconds serialization in some scenarios

[lmazuel]

No description provided.

[codecov-io]

Codecov Report

Merging #94 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #94      +/-   ##
=========================================
+ Coverage   29.09%   29.1%   +<.01%     
=========================================
  Files         380     380              
  Lines       16176   16178       +2     
=========================================
+ Hits         4707    4709       +2     
  Misses      11469   11469

Impacted Files	Coverage Δ
msrest/serialization.py	82.36% <100%> (+0.04%)	⬆️

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