Send sitemap.xml as a file rather than as content

Moving to minor.

Debugged this one and it doesn't make sense anymore. The sitemap.xml file is not a physical file, at least for simple_sitemap. It used to be sitemap files were added to the file system and we could do this but now they are created as routes so we should keep them as routes. Closing.

Reopening this. The XML file isn't rendering properly in the browser with the XSL because it's content type is:

text/html; charset=UTF-8

instead of:

application/xml; charset=utf-8

Update summary.

Had a hard time figuring out how to do this one, so marking postponed for now. We probably have to write the file ourselves and then send the file over.