fix: convert datetime back to proto for unit tests

[busunkim96]

Closes #508

field.mock_value (passed to the request) is a protobuf timestamp/duration, but the call gets a datetime/timedelta object. From Type Marshaling this seems to be the expected behavior.

Proto Plus provides a service that converts between protocol buffer objects and native Python types (or the wrapper > types provided by this library).

This allows native Python objects to be used in place of protocol buffer messages where appropriate. In all cases, we return the native type, and are liberal on what we accept.

I tried protobuf's ToDatetime() but the resulting datetime doesn't have tzinfo. https://googleapis.dev/python/protobuf/latest/google/protobuf/timestamp_pb2.html#google.protobuf.timestamp_pb2.Timestamp.ToDatetime

[codecov]

Codecov Report

Merging #511 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   100.00%   100.00%           
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  Files           26        26           
  Lines         1502      1502           
  Branches       308       308           
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  Hits          1502      1502           

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