Fix URI::build if scheme is file

[atalargo]

Hi
if a schema $id and $ref target a local file with file scheme (ex: 'file:///an/absolute/path/to/schema.json') the Uri::build method didn't add '//' after the 'file:' (because a local file uri, obviously, doesn't have a hostname component) so the ref loader after try to load 'file:/an/absolute/path/to/schema.json', and file_get_content didn't accept this.

I request this little fix for this case.

[sorinsarca]

Thank you for the patch! I'll merge this PR.