Embedded fallback URL is now requested URL

Merged #78
An SXG file has a fallback URL to be used when something wrong with the SXG file. But in previous implementation, it is modified to the actual URL and for example an access to https://example.com/ returns SXG embedding https://example.com/index.html.
It turned out to be an problematic behavior when user expect the fallback URL matches queried URL especially in the context of subresource fetching.
From now, request to https://example.com/ will return SXG with embedding URL https://example.com/ even though the actual contents exists in index.html.