fix: HTTP status responses for upserts

* PUT returns 201 instead of 200 when rows are inserted
* POST with "Prefer: resolution=merge-duplicates" returns 200 instead of 201 when no rows are inserted

src/PostgREST/ApiRequest/Preferences.hs

@@ -176,6 +176,7 @@ class ToHeaderValue a where
 data PreferResolution
   = MergeDuplicates
   | IgnoreDuplicates
+  deriving Eq
 
 instance ToHeaderValue PreferResolution where
   toHeaderValue MergeDuplicates  = "resolution=merge-duplicates"