Use system proxy settings instead of Proxy.NO_PROXY as default

[itmammoth]

When you call the api via the SlackApi object created without Proxy parameter, it tries to open the connection with Proxy.NO_PROXY(means direct connection).

System.setProperty("https.proxyHost","test.proxy.com"); // will be ignored
System.setProperty("https.proxyPort","8081"); // will be ignored
SlackApi api = new SlackApi(WEBHOOK_URL);
api.call(slackMessage); // open connection directly

I think this is not what most developers expect.