Fetch service should support request headers

[gabisurita]

There should be a way to pass headers (e.g. Authrization, KeepAlive, etc) as part of the request using fetch.

IMO the most idiomatic way would be to expose a fetch constructor (instead of having the fetch method already performing the request) and allow to include the javascript fetch API arguments as methods. I can try to sketch a PR if you agree with this pattern.

[therustmonk]

Thanks! It's resonable! I'm going to fix it.