Improve mutation determination

[justin-tay]

Description

Instead of attempting to guess if the query text is a mutation by checking the starting characters of the line parse the Document as the GraphQLEntityProjectionMaker is going to need it anyway to make the GraphQLProjectionInfo. It is then simple to check if the mutation operation is present in the Document.

Motivation and Context

This for instance allows handling of cases where a fragment exists before the mutation.

fragment postData on Post {
  id
  title
  text
  author {
    username
    displayName
  }
}
mutation addPost($post: AddPostInput!) {
  addPost(input: [$post]) {
    post {
      ...postData
    }
  }
}

How Has This Been Tested?

Modified the appropriate unit tests.

License

I confirm that this contribution is made under an Apache 2.0 license and that I have the authority necessary to make this contribution on behalf of its copyright owner.