Adjust serialization of timestamps within a QueryFilter to support DateTime and DateTimeOffset using the ISO-8601 (https://stackoverflow.com/a/115002)

Author: acupofjose

Postgrest/Attributes/ReferenceAttribute.cs

@@ -149,7 +149,7 @@ private void ParseColumns(ref List<ReferenceAttribute> seenRefs)
         }
 
         /// <inheritdoc />
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             if (obj is ReferenceAttribute attribute)
             {