This PR was merged into the 4.3-dev branch.

Discussion
----------

[Mime] added Headers::toArray()

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

That helps when using the Mime component to create a form submission HTTP body (where you need to merge the headers of the MIME body with the HTTP headers):

```php
$body->getPreparedHeaders()->toArray();

vs

foreach ($body->getPreparedHeaders()->getAll() as $header) {
      $headers[] = $header->toString();
}
```

Commits
-------

788bb81 [Mime] added Headers::toArray()