Extract ns from internaltype when available

[lmazuel]

Fix for Azure/azure-sdk-for-python#11568

[codecov-commenter]

Codecov Report

Merging #206 into master will increase coverage by 0.00%.
The diff coverage is 81.25%.

@@           Coverage Diff           @@
##           master     #206   +/-   ##
=======================================
  Coverage   83.77%   83.78%           
=======================================
  Files          25       25           
  Lines        2657     2664    +7     
=======================================
+ Hits         2226     2232    +6     
- Misses        431      432    +1     

Impacted Files	Coverage Δ
msrest/serialization.py	84.80% <81.25%> (+<0.01%)	⬆️

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