Support absolute URL requests

[twifkak]

https://tools.ietf.org/html/rfc7230#section-5.3 suggests that clients can make requests of the form GET http://host.example/path HTTP/1.1, though in practice they don't for typical clients/servers.

It seems that this line:

nginx-sxg-module/ngx_http_sxg_filter_module.c

Lines 702 to 703 in 97ea78b

	snprintf(fallback_url, fallback_url_length, "%s%s%.*s", kHttpsPrefix, host,
	(int)req->unparsed_uri.len, req->unparsed_uri.data);

doesn't support this type of request. It would construct an SXG with a fallback URL that looks like https://host.examplehttp://host.example/path.

TODO:

• Verify this is a problem (maybe it's not because unparsed_uri isn't as unparsed as its name implies?).
• Fix it.

Given the rarity of this case, it seems low-priority, and I'd recommend the easiest solution, e.g. don't generate a SXG when unparsed_uri is absolute.

[kumagi]

Compromising absolute URL preload is promising solution as libsxg does not support parsing SXG file yet.