Add set schema to middleware

Fix #828, breaking change: computed columns now only work if they are on
the config schema.

Fix #835, tests now not depend on the search_path of the
postgrest_test_authenticator.

test/Feature/QuerySpec.hs

@@ -171,6 +171,9 @@ spec = do
         [json| [{"id":1},{"id":2},{"id":3},{"id":4},{"id":5},{"id":6},{"id":7},{"id":8},{"id":9},{"id":10},{"id":11},{"id":12},{"id":13},{"id":14},{"id":15}] |]
         { matchHeaders = [matchContentTypeJson] }
 
+    it "cannot access a computed column that is outside of the config schema" $
+      get "/items?always_false=is.false" `shouldRespondWith` 400
+
     it "matches filtering nested items 2" $
       get "/clients?select=id,projects{id,tasks2{id,name}}&projects.tasks.name=like.Design*"
         `shouldRespondWith` [json| {"message":"Could not find foreign keys between these entities, No relation found between projects and tasks2"}|]