Err embedding when multiple relationships found

When having one-to-many relationships like:

person        -< message[sender]
person        -< message[recipient]
person_detail -< message[sender]
person_detail -< message[recipient]

Where person_detail is a view of person.

This request:

GET "/message?select=*,sender(*)"

Is ambiguous. Both person or person_detail could be embedded.

Until now we have returned the first detected relationship but
now we return a 300 Multiple Choices error with a
descriptive error message asking the user to disambiguate.

This is more helpful for the user and also aids in cases of more
complex relationships.

src/PostgREST/Types.hs

@@ -284,8 +284,8 @@ data Relation = Relation {
 , relLinkCols2 :: Maybe [Column]
 } deriving (Show, Eq)
 
-isSelfJoin :: Relation -> Bool
-isSelfJoin r = relTable r == relFTable r
+isSelfReference :: Relation -> Bool
+isSelfReference r = relTable r == relFTable r
 
 data PayloadJSON =
   -- | Cached attributes of a JSON payload