Simplify subscription JSON parsing

src/routes/api.cr

@@ -587,7 +587,7 @@ struct APIRouter
     post "/api/admin/plugin/subscribe" do |env|
       begin
         plugin_id = env.params.json["plugin"].as String
-        filters = JSON.parse(env.params.json["filters"].to_s).as_a.map do |f|
+        filters = env.params.json["filters"].as(Array(JSON::Any)).map do |f|
           Filter.from_json f.to_json
         end
         name = env.params.json["name"].as String