Handle empty enum values

[annatisch]

Fix for logged warnings where service returns an empty (None) value for an enum. These warnings were encountered during header deserialization.

[codecov-io]

Codecov Report

Merging #173 into master will decrease coverage by 0.07%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   87.84%   87.77%   -0.08%     
==========================================
  Files          25       25              
  Lines        2584     2584              
==========================================
- Hits         2270     2268       -2     
- Misses        314      316       +2

Impacted Files	Coverage Δ
msrest/polling/poller.py	100% <ø> (ø)	:arrow_up:
msrest/serialization.py	90.82% <100%> (-0.22%)	:arrow_down:

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[annatisch]

Yes there is a change in behaviour, but it's very small and I doubt it will affect any published SDKs. The change is that now missing deserialized enum headers will be None rather than "None".

If we want to preserve absolute behaviour, then this patch could be amended to:

if data is None:
    return str(data)
if isinstance(data, enum_obj):
    return data
# and so on....