Fix wrong URLs in browsable API

[lorey]

When developing under a port other than 80, URLs should be localhost:123/api but are localhost/api. Might have to do with nginx proxy, using USE_X_FORWARDED_HOST did not help so far.

[lorey]

Fixed it by using $http_host instead of $host. Not sure if that's optimal, but works for me. Explanation: https://serverfault.com/a/706439