Patch to add support for heart-beat

I am using stomp with ActiveMQ, and needed heart-beat support. Please consider merging my changes, which are available here:

https://github.com/atgreen/cl-stomp/commit/b6a4f3972ba6dbae196a28917e9098eb7ed4ebe5

I couldn't figure out how to fork and create a merge request on gitlab, so you'll have to grab it from github.

Thanks!