make zip with selector function private because
make zip with selector function private because

Scala collections do not have such a function, and we want to be
as close to Scala collections as possible

zhongdj/RxScala,jbripley/RxScala,hermanbanken/RxScala,zjrstar/RxScala,joohnnie/RxScala,jbripley/RxScala,ReactiveX/RxScala,samuelgruetter/RxScala,TracyLu/RxScala,samuelgruetter/RxScala,hermanbanken/RxScala