Previously, the heartbeat message, in its raw form, was ":\n\n". This
commit changes the message to be ":\n".
The former message, when parsed as Server-Sent Events, contained an
empty comment (as desired) _and_ a new line (erroneously). The new line
resulted in emitting any event that was presently being emitted, even if
it wasn't complete. That is, emitting an event partly, such as the
event's data but not its name. Removing the extra new line resolves this
issue and ensures that events aren't interrupted by the heartbeat.
Fixes#2152.
6fc24789e9