Uses selector to determine which source in sources to use. There is an alias
switchCase for browsers <IE9.
Function): The function which extracts the value for to test in a case statement.
Object): A object which has keys which correspond to the case statement labels.
Scheduler): The observable sequence that will be run if the sources are not matched. If this is not provided, it defaults to
Rx.Observabe.emptywith the specified scheduler.
Observable): An observable sequence which is determined by a case statement.