tests: don't match JSON bodies literally

The change of hash function with hashable-1.3.1.0 changes object
ordering in JSON output, causing some test failures:
https://app.circleci.com/pipelines/github/PostgREST/postgrest/805/workflows/067844c9-9ce4-49e8-8790-315625ab309b/jobs/8309
e.g.:

> expected: "[{\"b\":\"baz\",\"a\":\"bar\"}]"
>  but got: "[{\"a\":\"bar\",\"b\":\"baz\"}]"

This changes the tests to not compare against the body literally,
and instead use the ResponseMatcher instance from Test.Hspec.Wai.JSON.

test/SpecHelper.hs

@@ -8,7 +8,7 @@ import qualified Data.Set               as S
 import qualified System.IO.Error        as E
 
 import Data.Aeson           (Value (..), decode, encode)
-import Data.CaseInsensitive (CI (..))
+import Data.CaseInsensitive (CI (..), original)
 import Data.List            (lookup)
 import Data.List.NonEmpty   (fromList)
 import Network.Wai.Test     (SResponse (simpleBody, simpleHeaders, simpleStatus))
@@ -33,6 +33,12 @@ matchContentTypeJson = "Content-Type" <:> "application/json; charset=utf-8"
 matchContentTypeSingular :: MatchHeader
 matchContentTypeSingular = "Content-Type" <:> "application/vnd.pgrst.object+json; charset=utf-8"
 
+matchHeaderAbsent :: HeaderName -> MatchHeader
+matchHeaderAbsent name = MatchHeader $ \headers _body ->
+  case lookup name headers of
+    Just _  -> Just $ "unexpected header: " <> toS (original name) <> "\n"
+    Nothing -> Nothing
+
 validateOpenApiResponse :: [Header] -> WaiSession () ()
 validateOpenApiResponse headers = do
   r <- request methodGet "/" headers ""