Accept header overwritten on internal requests

I am using the Dispatcher to make internal API calls. One of them has the ability to return XML by checking the Accept header. This does work with internal requests since Dispatcher always sets the Accept after the defined headers have been set. So it gets overwritten.

`$this->dispatcher->header('Accept', 'application/xml')->header('foo', 'bar')->get('/api/v1/endpoint');`

```
array:6 [
"host" => []
"user-agent" => array:1 [
0 => "Symfony/3.X"
]
"accept" => array:1 [
0 => "application/x..v1+json" <-- Should be 'application/xml'
]
"accept-language" => array:1 [
0 => "en-us,en;q=0.5"
]
"accept-charset" => array:1 [
0 => "ISO-8859-1,utf-8;q=0.7,*;q=0.7"
]
"foo" => array:1 [
0 => "bar"
]
]
```

Problem would be solved by moving the separate line setting the default Accept header before adding the custom headers. The default would be still there, but a custom Accept header could be actually set. I can make a pull request if needed.

https://github.com/dingo/api/blob/master/src/Dispatcher.php#L481-L485