Deserialize Uris explicitly

Resolves #162 

I'm not sure if this is an upstream issue, but I've found that when we expect a Uri [here](https://github.com/octokit/octokit.net/blob/master/Octokit/SimpleJson.cs#L1374) we actually get a string - and then setting that value is just plain wrong.

No changes to the API.

cc @half-ogre for whether this should be a bandaid given the other serialization issues he's seen.