Fix mixed path separators in 'example/web-app/server' on Windows.

Windows allows either '/' or '\' as path separator though it forbids mixed path separators within one
fs request string. At the same time all URL requests use / in GET/HEAD/etc path.

As result example solution (web-app/server) creates mixed path strings and forwards them to fs.open or fs.readdir.

This commit adds conversion of path separators into OS required type.