Serde JSON serialization breaks for `Any` types without `#[serde(default)]`

[rnbguy]

Going through the serialize function, I realized, it calls try_unpack()

prost-wkt/wkt-types/src/pbany.rs

Lines 138 to 140 in 190d4d3

	match self.clone().try_unpack() {
	Ok(result) => serde::ser::Serialize::serialize(result.as_ref(), serializer),
	Err(_) => {

which may fail because "value" is assigned to {} in this block and the Message type may not derive #[serde(default)].

prost-wkt/wkt-types/src/pbany.rs

Lines 113 to 119 in 190d4d3

	pub fn try_unpack(self) -> Result<Box<dyn prost_wkt::MessageSerde>, AnyError> {
	let type_url = self.type_url.clone();
	let empty = json!({
	"@type": &type_url,
	"value": {}
	});
	let template: Box<dyn prost_wkt::MessageSerde> = serde_json::from_value(empty)

In such cases, serialization defaults to {"@type": ..., "value": ...}.

prost-wkt/wkt-types/src/pbany.rs

Lines 142 to 143 in 190d4d3

	state.serialize_field("@type", &self.type_url)?;
	state.serialize_field("value", &self.value)?;

This breaks the protobuf serialization to JSON for the Any types without #[serde(default)].