Fix assertion when requesting URL containing ':' from serveStaticFiles.

Paths containing a colon were considered to be Windows absolute
paths, causing Path.opBinary!("~") to assert. Now, the request is
just ignored (causing a 404 to be gennerated).
Merge pull request #694 from klickverbot/url-colon-crash

Fix assertion when requesting URL containing ':' from serveStaticFiles.