OperatorDematerialize

Operator Dematerialize

Issue #1060

Two notes:
- Since the dematerialize() is an instance method, there is no correct type parametrization for the operator. Therefore, I just used a raw version in the call.
- The original dematerialize could send out two onCompleted or an onError followed by an onCompleted because the materialize() always adds an onCompleted after the raw events. This affected operators due to unsafeSubscribe. This new version ensures this doesn't happen.