Some readable streams may only emit a 'close' event, and no 'end'
event. This patch fixes Stream#pipe so that it will still call
end() on the destination stream in that case.

Note: The test case for this patch also exposed another issue where
multiple pipes to the same writable stream will each call end() on
the source. This will break all remaining pipes.

When creating multiple .pipe()s to the same destination stream, the
first source to end would close the destination, breaking all remaining
pipes. This patch fixes the problem by keeping track of all open
pipes, so that we only call end on destinations that have no more
sources piping to them.