Stream aborts are not propagated

[ehmicky]

When all the input streams successfully end, so does the merged stream.
When any of the input streams errors, so does the merged stream.

However, when any of the input streams aborts (for example with stream.destroy()), the merged stream is not aborted. What's more, the merged stream never ends.

Generally speaking, finished() should be used, as it handles many edge cases of stream termination, which merge-streams does not correctly handle at the moment such as emitting the end event twice.