Make sure that flag enums with value zero are correctly serialized; fixes #640 #670

Job	Run time
windows-msvc-shared (CBOR)	9m 21s
windows-msvc-shared (JSON)	11m 54s
windows-msvc-shared (CAPNPROTO)	10m 55s
windows-msvc-shared (MSGPACK)	4m 50s
windows-msvc-shared (FLEXBUFFERS)	5m 25s
windows-msvc-shared (AVRO)	7m 20s
windows-msvc-shared (TOML)	4m 13s
windows-msvc-shared (UBJSON)	7m 10s
windows-msvc-shared (PARQUET)	37m 38s
windows-msvc-shared (YAML)	4m 42s
windows-msvc-shared (XML)	3m 55s
windows-msvc (JSON)	12m 42s
windows-msvc (MSGPACK)	3m 55s
windows-msvc (AVRO)	7m 21s
windows-msvc (XML)	4m 3s
windows-msvc (TOML)	4m 1s
windows-msvc (YAML)	4m 18s
windows-msvc (UBJSON)	7m 3s
windows-msvc (FLEXBUFFERS)	5m 49s
windows-msvc (benchmarks)	48m 57s
windows-msvc (CBOR)	9m 35s
windows-msvc (CEREAL)	4m 12s
windows-msvc (CAPNPROTO)	9m 40s
windows-msvc (PARQUET)	38m 41s
	4h 27m 40s

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