Interface: AppliesHeadersInterface  - X-Ref

Applies headers to a request.

This interface can be used with Guzzle streams to apply body specific
headers to a request during the PREPARE_REQUEST priority of the before event

NOTE: a body that implements this interface will prevent a default
content-type from being added to a request during the before event. If you
want a default content-type to be added, then it will need to be done
manually (e.g., using {@see GuzzleHttp\Mimetypes}).