Corrected a problem with siblingsId throwing an IndexError

[sonofmun]

An example of when this error is thrown can be seen in the test that I have added.
What is happening here is that the range_length is 7, end is 7, and len(document_reference) is 14. That means that end + range_length == len(document_references). That means that the elif statement does not fire and it goes to else, which errors because it tries to call document_references[end + range_length], which would be document_references[14], which doesn't exist. This change should correct the problem.

[coveralls]

Coverage remained the same at 94.116% when pulling a70eb96 on correction_siblingsId into 3ff0f27 on dev.

[PonteIneptique]

Sorry for the delay !