EZP-21678 Make unimplemented REST features return a 501 error
EZP-21678 CS issues
EZP-21678 changed to correct return in comments
Merge pull request #670 from StephaneDiot/impl/EZP-21678_Make_unimplemented_REST_features_return_a_501

Implement EZP-21678: Make unimplemented rest features return a 501 error code