feat: map scheme, host and port from elli

[albertocsouto]

Current erf implementation is hiding the values included in elli request scheme, host and port:

https://github.com/elli-lib/elli/blob/main/include/elli.hrl

Mapping doesn't work quite well for localhost, due an elli limitation:
elli-lib/elli#83

Maybe fixed if maintainer greenlights release 4.0.0:
elli-lib/elli#110