Add ability to map GUC to http response headers

src/PostgREST/Error.hs

@@ -10,6 +10,7 @@ module PostgREST.Error (
 , binaryFieldError
 , connectionLostError
 , encodeError
+, gucHeadersError
 ) where
 
 import           Protolude
@@ -79,6 +80,11 @@ binaryFieldError =
   simpleError HT.status406 [] (toS (toMime CTOctetStream) <>
   " requested but a single column was not selected")
 
+gucHeadersError :: Response
+gucHeadersError =
+ simpleError HT.status500 []
+ "response.headers guc must be a JSON array composed of objects with a single key and a string value"
+
 connectionLostError :: Response
 connectionLostError =
   simpleError HT.status503 [] "Database connection lost, retrying the connection."