Empty request no longer trigger fatal error.

[Loic-Rameau]

The middleware trigger a fatal error if the request body is empty, i came accros the problem so i came to share the resolution.

[Tucker-Eric]

I think the error may be before that when trying to call simplexml_load_string in the xml() method here: https://github.com/Tucker-Eric/laravel-xml-middleware/blob/master/src/XmlRequestServiceProvider.php#L27

Would you be able to update this pr to check for the content there and then return an empty object or array based on the $assoc paramater?

[Tucker-Eric]

Resolved in #7