Switching from "file" to special scheme when host is empty

[rmisev]

Sample:

var url = new URL('file:///path');
url.protocol = 'http:';

Current result: http:///path (with empty host); but after re-parse host gets "path" value.

I see two variants how to resolve this issue:

1. Change host to "localhost"
2. Or disallow such switch

Related to #259 / #269

[annevk]

I clearly didn't think hard enough. I will try to add this to the current PR. My tentative plan is to no-op for this case.

[annevk]

Done, added tests too.