Make `JsonFormat.Features` java.io.Serializable

[tiger9800]

JsonFormat.Features is a field of JsonFromat.Value, and we would like JsonFromat.Value to be java.io.Serializable. This was discovered in the process of implementing FasterXML/jackson-databind#5317.

Let's make the JsonFormat.Features java.io.Serializable

[cowtowncoder]

@tiger9800 Hmmh. I was under impression that all Java Enum types are always java.io.Serializable?

However: what is not is JsonFormat.Features, maybe you meant that type?

EDIT: tested, problem is indeed JsonFormat.Features.

[tiger9800]

My apologies, same mistake again. Thank you for changing the title. Changed the description too.

[cowtowncoder]

NP. Fixed, added a test to ensure no regression.