fix(react-native): add a default value for the withCredentials option

The undefined value breaks React Native on Android with a rather cryptic error message:

```
Attempt to invoke virtual method 'boolean java.lang.Boolean.booleanValue() on a null object reference
```

This bug was introduced by [1].

[1]: 5f47a50

Related: socketio/socket.io-client#1403

Author: darrachequesne

lib/socket.js

@@ -61,6 +61,7 @@ class Socket extends Emitter {
       {
         path: "/engine.io",
         agent: false,
+        withCredentials: false,
         upgrade: true,
         jsonp: true,
         timestampParam: "t",