Fix object input field shortcut as callable

[Vincz]

Fix error when using a callable as Input objects field type shortcut like this:

$inputObjType = new InputObjectType([
    'name'   => 'SomeInputObject',
    'fields' => [
        'f' => static function () : Type {
            return Type::string();
        },
    ],
]);

[coveralls]

Coverage remained the same at 86.38% when pulling 4b5354e on Vincz:master into 115a20e on webonyx:master.

[spawnia]

Looks good to me

[spawnia]

It would be great if you could add a line to the CHANGELOG.md

[Vincz]

@spawnia Hi! Changelog has been updated as requested.