[BrowserKit] Fixed relative redirects for ambiguous paths

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |

This fixes a problem I discovered today during Mink testing: I have a route with a slug and id, separated by colon (`/{slug}:{id}`). When the `HTTP_HOST` is being normalized it only checks for array key existence, not emptyness. In this case it was `false` due to the `parse_url` call in `updateServerFromUri`, which cannot handle this case:

```
$> php -r "var_dump(parse_url('/redirect', PHP_URL_HOST));"                                                                               NULL

$> php -r "var_dump(parse_url('/redirect:1234', PHP_URL_HOST));"                                                                          bool(false)
```

So now the url becomes `http:///redirect:1234`, because of the missing host.