[BrowserKit] fixed protocol-relative url redirection

Fixed redirects to protocol relative URLs, e.g. //www.example.org.  Previously the code would treat this as a relative URL.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9445
| License       | MIT
| Doc PR        |