Response Object is empty if there is no given AccessToken on Resource

The Response Object has empty messages if the client access a restricted resource without providing an Access Token.

Test:
```
http://localhost/resource.php
```
Client can connect using an Access Token as: 
```
http://localhost/resource.php?access_token=xxxxxx
```

If it ask for it without providing the param "access_token" the Response Object replies: 
```
[]
```

OAuth2 standard dictates that you should reply with some kind of information (http://tools.ietf.org/html/rfc6749#section-7.2).

With this patch the server replies:
```json
{
error: "invalid_token",
error_description: "Protected resource and no access token provided."
}
```