Return request relative link headers, fixes #4925

When WordPress is running inside of a sub-directory,
get_woocommerce_api_url() returns the domain along with the
sub-directory path, which results in a duplicate path when the API
request string is added. This commit fixes this by fetching only the
home host and adding the request path to it. set_url_scheme() is used
to set the scheme to either http or https based on the request itself
instead of basing it on whether force_ssl_checkout is enabled or not.