Fix #233: handle nil headers (#234)
Fix #233: handle nil headers (#234)

#233 introduced a bug, making the SDK crashes when WebSocket is invoked without HTTP headers/with nil headers, since we try to dereference a nil value.
This PR fixes the problem by adding a guard against nil HTTP headers.

* [websocket] allow passing HTTP headers to the ctor
* [bugfix] do not dereference a nil value

kuzzleio/sdk-go,kuzzleio/sdk-go