Support concat of an already-flowing stream

Per your request in issue #87, this pull request adds a test for concat of an already-flowing stream (e.g., concatting an existing pipeline).  Without the changes to ``lib/index.js``, the test will fail, by never calling ``test.done()``.

All tests (as run by ``npm test``) still pass with eager piping, as implemented in this patch.  If there's something that breaks due to eager piping, it's not covered by the tests as far as I can tell.