Aggregations: Added getProperty method to Aggregations

This allows arbitrary properties to be retrieved from an aggregation tree. The property is specified using the same syntax as the
order parameter in the terms aggregation. If a property path contians a multi-bucket aggregation the property values from each bucket will be returned in an array.
I see... But then it is not consistent with other aggs that return themselves when the path is empty? 

Would it solve the issue to modify ordering so that it calls value() when the path point to a single-value agg since there is no ambiguity in that case?