return promise from setState endpoint

[jskrzypek]

The changelog for v4.2.0 lists "Add optional support for promises on all endpoints."

The documentation for what returns promises is basically non-existent, but I received an error when trying to do this:

pubnub.setState({ channels: ['my_channel'], state: { 'isTyping': 'true' } }).then((res) => { console.log(res); });

The error I get is "EXCEPTION: Cannot read property 'then' of undefined", because pubnub.setState() doesn't return a promise like the hereNow(), whereNow(), and getState() endpoints.

After digging into the source a bit, I realized that the reason the function is not returning the promise is that the call to the endpoint built by the util is not actually being returned by _subsctiptionManager.adaptStateChange(), here, which should instead be this:

return this._setStateEndpoint({ state, channels, channelGroups }, callback);

I also added an integration test to stateSetting.test.js to ensure that the promises succeed.

[MaxPresman]

merged. a gulp build was required to generate the library files but it's all covered! Thank you for the PR!

[MaxPresman]

7044017