Update server.ts

It's odd to compare the length with 0. In the original code, I need to do this to add a custom header:

let custom_header = {};
let server = new Server('https://url_to_server', {headers: custom_header});
custom_header['MyHeader'] = 'Some header content';

Shouldn't it check for non-zero to add the custom header?

src/rpc/server.ts

@@ -75,7 +75,7 @@ export class Server {
   constructor(serverURL: string, opts: Server.Options = {}) {
     this.serverURL = URI(serverURL);
 
-    if (opts.headers && Object.keys(opts.headers).length === 0) {
+    if (opts.headers && Object.keys(opts.headers).length !== 0) {
       AxiosClient.interceptors.request.use((config: any) => {
         // merge the custom headers into any existing headers
         config.headers = Object.assign(config.headers, opts.headers);