[FIX] Allow fallback file to work when passing in multiple root directories

[jackp]

Since the fallback file gets set after every directory is served, the request will never be able to hit any subsequent directories.

This PR sets the fallback files at the end of the stream, therefore allowing users to use

gulp.src(['web', 'tmp'])
.pipe(webserver({}))

[jackp]

This feature is working fine for me, but I'm having trouble getting the test to pass

[schickling]

Thanks @jackp for your PR. Please fix the tests, I want to keep the CI env healthy :+1: