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.

postgrest.cabal

@@ -127,6 +127,7 @@ test-suite spec
                       Feature.ConcurrentSpec
                       Feature.CorsSpec
                       Feature.DeleteSpec
+                      Feature.EmbedDisambiguationSpec
                       Feature.ExtraSearchPathSpec
                       Feature.InsertSpec
                       Feature.JsonOperatorSpec