url: fix resolving from non-file to file URLs.

When resolving a reference URL with the 'file' scheme an no host
against a base URL without the 'file' scheme, the first path element
of the reference URL is used as the host for the target URL. This
results in an invalid target URL.

This change makes an exception for file URLs so that the host is not
mangled during URL resolution.

PR-URL: https://github.com/iojs/io.js/pull/1277


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Petka Antonov <petka_antonov@hotmail.com>