Fix exception when sending multiple messages

Looks like this has been broken since
07c16db72b79c409c3db3775c2b45cf066b51585, which added a `room` parameter
to `Adapter::send`. dc27aca6fcbd0d5b8d7fdc47f2feeb6638d3a028 changed
that to an `envelope` parameter, but didn't quite fix the bug.
Merge pull request #503 from aroben/send-multiple

Fix exception when sending multiple messages