Prefer to read url from HTTP_HOST than SERVER_NAME

Summary:
The nginx web server allows to use an invalid domain name,
like _ as a server name.

In such case, we can't deduce a lot of things from the canonical
server name defined by the web server. HTTP_HOST is more informative.

Ref T1294

Reference: https://nginx.org/en/docs/http/server_names.html

Test Plan: On nginx, create a server block with server_name _;

Reviewers: dereckson

Reviewed By: dereckson

Tags: Keruald

Maniphest Tasks: T1294

Differential Revision: https://devcentral.nasqueron.org/D1168