Make sure RFC dates are pickable

[lmazuel]

Fix #205

[codecov-commenter]

Codecov Report

Merging #208 into master will increase coverage by 0.03%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   83.78%   83.81%   +0.03%     
==========================================
  Files          25       25              
  Lines        2664     2669       +5     
==========================================
+ Hits         2232     2237       +5     
  Misses        432      432              

Impacted Files	Coverage Δ
msrest/serialization.py	84.87% <81.25%> (+0.07%)	⬆️

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