[BrowserKit] Allow URLs that don't contain a path when creating a cookie from a string

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The Cookie class doesn't require a path, but the `fromString()` static method does, so when URL given is something like http://www.example.com (ie no trailing slash) it currently throws an exception. This PR removes the requirement.