Add option for overriding root spec (#1317)

* Only for pg >= 9.6

* Disallow specifying schema on root-spec

* Increase memory test upper bound

test/Feature/RootSpec.hs

@@ -0,0 +1,30 @@
+module Feature.RootSpec where
+
+import Network.HTTP.Types
+import Network.Wai        (Application)
+
+import Test.Hspec
+import Test.Hspec.Wai
+import Test.Hspec.Wai.JSON
+
+import Protolude hiding (get)
+
+import SpecHelper
+
+spec :: SpecWith Application
+spec =
+  describe "root spec function" $ do
+    it "accepts application/openapi+json" $
+      request methodGet "/"
+        [("Accept","application/openapi+json")] "" `shouldRespondWith`
+        [json|{
+           "swagger": "2.0",
+           "info": {"title": "PostgREST API", "description": "This is a dynamic API generated by PostgREST"}
+          }|]
+        { matchHeaders = ["Content-Type" <:> "application/openapi+json; charset=utf-8"] }
+
+    it "accepts application/json" $
+      request methodGet "/"
+        [("Accept", "application/json")] "" `shouldRespondWith`
+        [json| [{"table": "items"}, {"table": "subitems"}] |]
+        { matchHeaders = [matchContentTypeJson] }