Mutations aren't checked

[MichaelJCompton]

Currently mutations aren't check for correctness.

For example, a mutation using the AuthorPosts example, adds a post with

{
  "input": {
    ...
    "author": { "uid": "0x22" }
  }
}

where 0x22 is the uid of say another post, rather than an author, will succeed and corrupt the data.

This should be failed as a type checking error in the data.