Forbid or replace empty targets in HTTP/1.1 requests

[ruslandoga]

👋

Right now Mint allows sending empty target in the request line for HTTP/1.1. I think it should either forbid such requests or replace them with /:

If the target URI's path component is
empty, the client MUST send "/" as the path within the origin-form of
request-target.

Current behaviour

Mint.HTTP1.request(conn, "POST", _path = "", headers, body)

this results in POST HTTP/1.1

This PR

Forbidding seems easier since replacing "" with "/" might be surprising and not always correct, and there is already a :skip_target_validation option to disable "forbidding" if necessary.

[whatyouhide]

@ruslandoga can we add a test that verifies the behavior of this with :skip_target_validation?