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/Query/SqlFragment.hs

@@ -24,6 +24,8 @@ module PostgREST.Query.SqlFragment
   , fromJsonBodyF
   , responseHeadersF
   , responseStatusF
+  , addConfigPgrstInserted
+  , currentSettingF
   , returningF
   , singleParameter
   , sourceCTE
@@ -433,6 +435,11 @@ responseHeadersF = currentSettingF "response.headers"
 responseStatusF :: SQL.Snippet
 responseStatusF = currentSettingF "response.status"
 
+addConfigPgrstInserted :: Bool -> SQL.Snippet
+addConfigPgrstInserted add =
+  let (symbol, num) =  if add then ("+", "0") else ("-", "-1") in
+  "set_config('pgrst.inserted', (coalesce(" <> currentSettingF "pgrst.inserted" <> "::int, 0) " <> symbol <> " 1)::text, true) <> '" <> num <> "'"
+
 currentSettingF :: SQL.Snippet -> SQL.Snippet
 currentSettingF setting =
   -- nullif is used because of https://gist.github.com/steve-chavez/8d7033ea5655096903f3b52f8ed09a15