Serialization round-trip fails for floating-point numbers if they're serialized to `null`

[vpanteleev-sym]

For example, the default value of double (which is a NaN) is serialized to null, which throws an error when attempting to deserialize:

/+dub.sdl:
 dependency "vibe-serialization" version="*"
+/
import std.stdio;
import vibe.data.json;

void main() {
	double d;
	d.serializeToJsonString                       .writeln; // prints "null"
	d.serializeToJsonString.deserializeJson!double.writeln; // throws
}

I think serialization libraries should try to ensure that a serialization and deserialization round-trip does not have such surprises.

We could do one of:

• Throw on serialization if the value is not deserializable;
• Serialize as a string (e.g. "NaN") and recognize it when deserializing
• If it's just NaN, then understand null as NaN (but not sure about infinities).