Avoid using deprecated Buffer constructor

safe-buffer is already used and provides Buffer.from polyfill, so
just use Buffer.from directly to avoid hitting deprecated API.

Refs:
https://nodejs.org/api/deprecations.html#deprecations_dep0005_buffer_constructor

lib/spdy-transport/stream.js

@@ -359,7 +359,7 @@ Stream.prototype._onFinish = function _onFinish () {
       id: this.id,
       priority: state.priority.getPriority(),
       fin: true,
-      data: new Buffer(0)
+      data: Buffer.alloc(0)
     })
   }