Serialize sync_events::Filter untagged

This is to make Filter::FilterId(String::from("0")) serialize
as "0", instead of to a JSON object that cannot be passed
as a query paramater

src/r0/sync.rs

@@ -260,6 +260,7 @@ pub mod sync_events {
 
     /// A filter represented either as its full JSON definition or the ID of a saved filter.
     #[derive(Clone, Debug, Deserialize, Serialize)]
+    #[serde(untagged)]
     pub enum Filter {
         /// A complete filter definition serialized to JSON.
         FilterDefinition(FilterDefinition),