Spanner should know how to merge chunk data of TIMESTAMP and DATE type

[yixizhang]

https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/spanner/google/cloud/spanner/streamed.py#L300

The dict doesn't cover type_pb2.TIMESTAMP and type_pb2.DATE, the fix is actually easy because both type is passed in string_value.

[vkedia]

Thanks for filing this. This indeed looks like a bug.
@tseaver @lukesneeringer please take a look.