Make sure that flag enum with value zero are correctly serialized; fi… #669

Job	Run time
windows-msvc-shared (XML)	4m 32s
windows-msvc-shared (CAPNPROTO)	7m 49s
windows-msvc-shared (PARQUET)	39m 5s
windows-msvc-shared (JSON)	11m 29s
windows-msvc-shared (MSGPACK)	4m 19s
windows-msvc-shared (FLEXBUFFERS)	5m 20s
windows-msvc-shared (YAML)	4m 27s
windows-msvc-shared (AVRO)	7m 28s
windows-msvc-shared (UBJSON)	7m 3s
windows-msvc-shared (CBOR)	8m 34s
windows-msvc (UBJSON)	7m 21s
windows-msvc (FLEXBUFFERS)	5m 31s
windows-msvc (XML)	4m 33s
windows-msvc (AVRO)	7m 36s
windows-msvc (CBOR)	9m 17s
windows-msvc-shared (TOML)	4m 10s
windows-msvc (CEREAL)	4m 10s
windows-msvc (benchmarks)	49m 0s
windows-msvc (CAPNPROTO)	8m 20s
windows-msvc (JSON)	12m 28s
windows-msvc (MSGPACK)	3m 58s
windows-msvc (PARQUET)	39m 37s
windows-msvc (TOML)	4m 13s
windows-msvc (YAML)	4m 44s
	4h 25m 4s

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