Fetching relationship already included attempts to get relationship from server

[BryanHunt]

If you get a resource and set { include: ["author] } and then later call book.fetch("author"), the client will not use the included author, but instead attempt to get it from the server. I discovered this because it hits Issue #69

[paparomeo]

Thank you for raising this issue. We'll try to fix it at some point, but please feel free to submit a PR with a fix which would be most welcome.