[WebProfiler] Can't consult the exception panel when there is an exception

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no (not yet)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | not reported yet AFAIK
| License       | MIT
| Doc PR        | none

When consulting the Exception Panel of the Web Profiler, there was an exception stating that the ResponseHeaderBag expects an array as a typehint, and it has a string instead

```
An exception has been thrown during the rendering of a template ("Catchable Fatal Error: Argument 1 passed to Symfony\Component\HttpFoundation\ResponseHeaderBag::__construct() must be of the type array, string given
``` 

This was induced in the commit 6d2135b.

The thing is, as I already discussed on IRC with @lsmith77, I think it is strange that the `Response`'s constructor does not have a typehint on the `$headers` parameter, as it is directly fed to the `ResponseHeaderBag`, on the very first instruction of the mentionned controller, which provokes the mentionned error. So either we can add the typehint on the Response, or we can also typehint the parameter :

```php
<?php

class Response
{
    // ...
    public function __construct($content = '', $status = 200, $headers = array())
    {
        $this->headers = new ResponseHeaderBag((array) $headers);
    }

    // ...
}
```

Though I think it is not as neat as the typehint directly on the parameter, but  I heard that it was a no-go to add a typehint on object's constructor, as this could be a potential BC Break. Is it still the case ? Even though this call are here for at least 2 years now...

Cheers.