only close websocket once

[wiegell]

Fixes #2604

I have:

• Removed the defer c.close(websocket.CloseAbnormalClosure, "unexpected closure") since c.close() is also triggered via go c.closeOnCancel(ctx) on line 281. This seems to have fixed the dual firing of a supplied closefunction for me, where the exit of the for loop on line 291 would fire the close function twice (via the defer directly, but also via go c.closeOnCancel(ctx) indirectly triggered by the defer). I will however point to line 305 and line 317, where the loop is also exited with explicit closures. I have not run into those lines in the debugger, but i would actually have expected those places to fire c.close() three times before, and now instead two times (probably still too many).
  Let me know if i can add this expected behavior to existing tests.

[coveralls]

coverage: 74.454% (-0.04%) from 74.498%
when pulling 2f16898 on wiegell:master
into 256794a on 99designs:master.

[StevenACoffman]

Thanks!