More generic wrapper for singleton decorator

[cyrilbouvier]

diff --git a/half_orm/relation.py b/half_orm/relation.py
index 7ef16f5..bf4f830 100644
--- a/half_orm/relation.py
+++ b/half_orm/relation.py
@@ -906,12 +906,12 @@ def singleton(fn):
     _is_singleton is unset as soon as a Field is set.
     """
     @wraps(fn)
-    def wrapper(self):
+    def wrapper(self, *args, **kwargs):
         if self._is_singleton:
-            return fn(self)
+            return fn(self, *args, **kwargs)
         try:
             self = self.get()
-            return fn(self)
+            return fn(self, *args, **kwargs)
         except relation_errors.ExpectedOneError as err:
             raise relation_errors.NotASingletonError(err)
     return wrapper