bug #42416 Invalid DKIM signature (metaer)

This PR was merged into the 5.4 branch.

Discussion
----------

Invalid DKIM signature

| Q             | A
| ------------- | ---
| Branch?       | 5.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| License       | MIT

How to reproduce:

```php
$email = (new Email())
    ->from('[email protected]')
    ->subject('text')
    ->text('text')
;

$addresses = '[email protected],[email protected]' //could be command's or method's argument, that's why we used call_user_func_array below
\call_user_func_array([$email, 'to'], explode(',', $addresses));

$email->getHeaders()->addTextHeader('X-Transport', $transport);
$privateKeyFilePath = '/private.pem';
$filesystem = new Filesystem();

if ($filesystem->exists($privateKeyFilePath)) {
    $signer = new DkimSigner("file://$privateKeyFilePath", 'yourdomain.com', 'your-selector');
    $email = $signer->sign($email, ['headers_to_ignore' => ['x-transport']]); //symfony/symfony#39354 (comment)
}

$this->mailer->send($email);
```

How I tested body hash (bh): https://www.appmaildev.com/site/testfile/dkim?lang=en
It shows expected body hash (`Expected-Body-Hash`) in DKIM section and received body hash (`bh=`) if fails.

![image](https://user-images.githubusercontent.com/6103208/128579511-ead9fde9-1492-4a6a-8953-2adb99eb251a.png)

My solution is based on swiftmailer signer code and DKIM specifications (see links below).
After applying patch it works correctly:
![image](https://user-images.githubusercontent.com/6103208/128579683-ba1599f4-956c-49f6-ba94-f69bb8979c8d.png)

How swiftmailer signer adds trailing line return:
```php
    protected function endOfBody()
    {
        // Add trailing Line return if last line is non empty
        if (\strlen($this->bodyCanonLine) > 0) {
            $this->addToBodyHash("\r\n");
        }
        $this->bodyHash = hash_final($this->bodyHashHandler, true);
    }
```
Swiftmailer signer works correctly with not empty body.

From DKIM signature specifications:

- simple canonicalization: https://datatracker.ietf.org/doc/html/rfc6376#section-3.4.3 `If there is no body or no trailing CRLF on the message body, a CRLF is added`
- relaxed canonicalization: https://datatracker.ietf.org/doc/html/rfc6376#section-3.4.4 `If the body is non-empty but does not end with a CRLF, a CRLF is added`

Other issues related to invalid DKIM signature: #39354, #41935, #42407. But is seems they have another problem, which is connected to templated emails.

I have tested dkim signature manually (with gmail) with these cases:

canonicalization: simple
body: ''

canonicalization: simple
body: "\r\n"

canonicalization: simple
body: 'text'

canonicalization: relaxed
body: ''

canonicalization: relaxed
body: "\r\n"

canonicalization: relaxed
body: 'text'

![image](https://user-images.githubusercontent.com/6103208/128577659-1e765fe3-19f7-4ef2-bd59-22064e0d9b73.png)

Commits
-------

0cb61f4828 Add trailing Line return if last line is non empty

Author: fabpot

Crypto/DkimSigner.php

@@ -203,7 +203,13 @@ private function hashBody(AbstractPart $body, string $bodyCanon, int $maxLength)
             hash_update($hash, $canon);
         }
 
-        if (0 === $length) {
+        // Add trailing Line return if last line is non empty
+        if (\strlen($currentLine) > 0) {
+            hash_update($hash, "\r\n");
+            $length += \strlen("\r\n");
+        }
+
+        if (!$relaxed && 0 === $length) {
             hash_update($hash, "\r\n");
             $length = 2;
         }

Tests/Crypto/DkimSignerTest.php

@@ -122,14 +122,14 @@ public function getCanonicalizeHeaderData()
             DkimSigner::CANON_SIMPLE, "\r\n", '', \PHP_INT_MAX,
         ];
         yield 'relaxed_empty' => [
-            DkimSigner::CANON_RELAXED, "\r\n", '', \PHP_INT_MAX,
+            DkimSigner::CANON_RELAXED, '', '', \PHP_INT_MAX,
         ];
 
         yield 'simple_empty_single_ending_CLRF' => [
             DkimSigner::CANON_SIMPLE, "\r\n", "\r\n", \PHP_INT_MAX,
         ];
         yield 'relaxed_empty_single_ending_CLRF' => [
-            DkimSigner::CANON_RELAXED, "\r\n", "\r\n", \PHP_INT_MAX,
+            DkimSigner::CANON_RELAXED, '', "\r\n", \PHP_INT_MAX,
         ];
 
         yield 'simple_multiple_ending_CLRF' => [