allow writes when pending

[jamesthomp]

Thanks for this library. I've tried to use it with another library which essentially does:

const net = require("net");
const client = net.createConnection({ port: 1883, host: "example.com" });
client.write("Hello, server!");

The above code works in node without raising an error, but in this library it throws the 'Socket is closed.' error. This change resolves the issue and queues up the writes until the socket connects.