Wiki Export sample Update

Forgot to send a pull request for this before moving to the new workspace. Noticed it on the old one. 

Fixes wikiPage getcontent, adds html content to wiki export sample.
I was thinking that we could get rid of this method and of the need to pass this parameter at all, if we just add that into the UrlBuilder.format() method, where we already have the service, and we can get the endpoint from the service, and the isAuthenticated from the endpoint.