Only send messages when socket is open

[pandeiro]

Currently adzerk.boot-reload.websocket/transmit fires .send unconditionally; checking that the socket is open with adzerk.boot-reload.websocket/open? first would only send messages when the socket is actually open.

The use-case for this is when boot-reload is loaded in browser environments (e.g. headless browsers) that may not have full websocket support, for instance in testing. Since boot-cljs doesn't currently support multiple :optimizations :none builds (and boot-reload would try to add itself to each .cljs.edn spec anyway, as currently written), this change would facilitate doing both development and testing with a single build even if it includes boot-reload.

[martinklepsch]

Closed by #26