Set explicit Host header for HTTP.jl v2 compatibility

[pebeto]

Adds the runtime fix needed to actually make HTTP.jl v2 work against the OpenML API, complementing the compat bump in #34.

Root cause

HTTP.jl v2 writes the port into the Host header for HTTPS requests, e.g. Host: www.openml.org:443. OpenML's server rejects any request whose Host carries an explicit port — it closes the connection, which surfaces as:

http parse error: unexpected EOF while reading HTTP/1 data

HTTP.jl v0.x/v1.x and curl omit the default port, which is why this only appears on v2.

Fix

Set the Host header explicitly (without the port), derived once from API_URL.

[jbrea]

Thanks!