DATAREST-219: Write deleteProperty test case and then simplify delete operation

After coding a test case against the existing behavior, used https://github.com/spring-projects/spring-data-rest/pull/86
as inspiration to rewrite the delete property operation and avoid creating a new collection.
Instead, using an iterator, the propertId is removed from the collection before it
is re-saved. Also did the same with MapLike properties by iterating over the key set.