[Security] Fix BCryptPasswordEncoder to encode password using salt

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

After changing the bcrypt logic from Symfony to `ircmaxell/password-compat` in b2e553a the salt was never passed to the password_hash method. 

This actually still occurs in 9d089ef as well:

```php
if (function_exists('password_hash')) {
   return password_hash($raw, PASSWORD_BCRYPT, array('cost' => $this->cost));
}
```
I don't know if there wasn't salt support previously which is why it was never implemented before. Since a lot of the logic was changed in the 2.3 branch in the class BCryptPasswordEncoder i thought it would be better to issue a pull request here.