DateTime fractions of seconds are lost

[PhilippJust]

Expected Behavior

• When inserting an entity with DateTime properties to a table, I expect the stored value to have the same precision as the property of the entity

Actual Behavior

• Fractions of seconds are only present for the standard Timestamp column but lost for custom properties.
  See this screenshot taken with the azure storage explorer
  

Proposed solution

The transformation is defined in the azure storage common project in line 46 of _serialization.py.

def _to_utc_datetime(value):
    # Azure expects the date value passed in to be UTC.
    # Azure will always return values as UTC.
    # If a date is passed in without timezone info, it is assumed to be UTC.
    if value.tzinfo:
        value = value.astimezone(tzutc())
    return value.strftime('%Y-%m-%dT%H:%M:%SZ')

It is seen that this function always ignores everything smaller than a second. So therefor I propose to change this line to include milliseconds as well.