Make WebSocket constructor parameter "protocol" optional

In the type declarations and official spec this parameter is optional.

Change-Id: Idaf35b6fa7859c0eb95b5dc998a2a6e7a7d4f4fe

Author: tbuschto

src/tabris/WebSocket.js

@@ -37,7 +37,7 @@ NativeObject.defineProperties(_WebSocket.prototype, {
 
 export default class WebSocket {
 
-  constructor(url, protocol) {
+  constructor(url, protocol = '') {
     if (typeof url !== 'string') {
       throw new Error(`WebSocket url ${toValueString(url)} is not a string`);
     }

test/tabris/WebSocket.test.js

@@ -24,10 +24,6 @@ describe('WebSocket', function() {
       expect(WebSocket).to.be.a('function');
     });
 
-    it('declares 2 formal parameters', function() {
-      expect(WebSocket.length).to.equal(2);
-    });
-
     it('fails when url parameter is omitted', function() {
       expect(() => new WebSocket())
         .to.throw('WebSocket url undefined is not a string');
@@ -43,9 +39,8 @@ describe('WebSocket', function() {
         .to.throw('WebSocket url has to have a scheme of \'ws\' or \'wss\' but is \'http\'');
     });
 
-    it('fails when protocol is omitted', function() {
-      expect(() => new WebSocket('ws://url.com'))
-        .to.throw('WebSocket protocol has too be a string or an array of strings');
+    it('accepts missing protocol', function() {
+      expect(() => new WebSocket('ws://url.com')).not.to.throw(Error);
     });
 
     it('fails when protocol is not string', function() {