Fix (de)serialization for filter in get_context

src/r0/context/get_context.rs

@@ -35,7 +35,11 @@ ruma_api! {
 
         /// A RoomEventFilter to filter returned events with.
         #[ruma_api(query)]
-        #[serde(skip_serializing_if = "Option::is_none")]
+        #[serde(
+            with = "ruma_serde::json_string",
+            default,
+            skip_serializing_if = "Option::is_none"
+        )]
         pub filter: Option<RoomEventFilter>,
     }