Authentication header preference

Is there a preference to the headers that are provided to AppSync? In the event multiple headers are sent, which one takes precedence?

```
headers: {
x-api-key: 'XXXXXX',
authorization: 'XXXXXX'
}
```

It was my experience all the way up until this past friday that the `Authorization` header was preferred, but now I'm getting things like

```
"message": "Not Authorized to access me on type Query"
```

When sending the same queries after friday. If there is a documentation on this, I'd really love to see it. I've searched for a while and couldn't find anything.

Start by reviewing the AWS AppSync authentication documentation and the issue's examples of x-api-key and authorization headers. Document which credential takes precedence when both are supplied, and confirm that the documented behavior explains the reported Not Authorized error.

Written by the indexing model from the issue text.