Add more flexibility to conservation policy update

src/data/base_request_handler.rs

@@ -357,7 +357,7 @@ pub trait BaseRequestHandler {
     }
 
     fn post_positive_u64_opt(&mut self, name: &str) -> Res<Option<u64>> {
-        match self.post_u64(name)? {
+        match self.post_u64_opt(name, 0)? {
             0 => Ok(None),
             val => Ok(Some(val))
         }