fix: stop assigning global WebSocket
fix: stop assigning global WebSocket

Previously, globel.WebSocket was assigned accidentally, which caused crashs in React Native as global.WebSocket is readonly.

fix #281

leancloud/js-realtime-sdk,leeyeh/js-realtime-sdk,leeyeh/js-realtime-sdk,leeyeh/js-realtime-sdk,leancloud/js-realtime-sdk