Add check to ensure serialization code only added when underlying value can be serialized.

source/handy_http_primitives/optional.d

@@ -88,6 +88,9 @@ struct Optional(T) {
     version (Have_asdf) {
         import asdf;
 
+        static if (
+            __traits(compiles, deserialize!T(Asdf.init))
+        ) {
             /**
             * Deserializes an optional value from raw ASDF data. This function
             * is defined to allow for automatic deserialization of Optionals when
@@ -122,6 +125,7 @@ struct Optional(T) {
             }
         }
     }
+}
 
 /**
  * Maps the value of a given optional to another type using a given function.