Include millisecond component (%f) in datetime serialization

[gr-rob]

azure-storage-python/azure-storage-common/azure/storage/common/_serialization.py

Line 46 in 4306898

return value.strftime('%Y-%m-%dT%H:%M:%SZ')

Milliseconds are being stripped from datetime values when serializing:

return value.strftime('%Y-%m-%dT%H:%M:%SZ')
-->
return value.strftime('%Y-%m-%dT%H:%M:%S.%fZ')