Align single() interface to last() to allow emit default value?

[kwonoj]

In case of last operator, it's allows to specify default value if there isn't any matching element.

Rx.Observable.prototype.last([predicate], [thisArg], [defaultValue])

single does similar jobs as operator that returns first matching element instead of last, but it does not allows to specify default value as

Rx.Observable.prototype.single(predicate, [thisArg])

so if there isn't any matching element only undefined will be emitted.

Maybe it'd be better to align single also can accept default value?
Initial PR mimics existing behavior (do not accept default value) is created at #322

[kwonoj]

At second thought, feeling it may be better to leave single as is since first does jobs for default value and single specifically guarantees if there is only single matching element. Leave issue opened to get clarification for now.

[benlesh]

I'm closing this for now, as I think when it was asked, there was some confusion about the behavior of single.