minor #16428 [Mailer] Capitalise From and Bcc header names in global mailer configuration (andyexeter)

This PR was submitted for the 5.4 branch but it was merged into the 5.3 branch instead.

Discussion
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[Mailer] Capitalise From and Bcc header names in global mailer configuration

If the `From` header name is not capitalised, the header is replaced by the `Sender` address in the Mime component's Message object:
 https://github.com/symfony/symfony/blob/6a52b66da0d35b21031bb344e79522f73d60e05a/src/Symfony/Component/Mime/Message.php#L76-L81

I capitalised `Bcc` as well because the component also looks for this header in a case-sensitive way.

Commits
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418426a Capitalise From and Bcc header names in global mailer configuration

Author: javiereguiluz

mailer.rst

@@ -535,8 +535,8 @@ and headers.
                     sender: '[email protected]'
                     recipients: ['[email protected]', '[email protected]']
                 headers:
-                    from: 'Fabien <[email protected]>'
-                    bcc: '[email protected]'
+                    From: 'Fabien <[email protected]>'
+                    Bcc: '[email protected]'
                     X-Custom-Header: 'foobar'
 
     .. code-block:: xml
@@ -558,8 +558,8 @@ and headers.
                         <framework:recipients>[email protected]</framework:recipients>
                         <framework:recipients>[email protected]</framework:recipients>
                     </framework:envelope>
-                    <framework:header name="from">Fabien &lt;[email protected]&gt;</framework:header>
-                    <framework:header name="bcc">[email protected]</framework:header>
+                    <framework:header name="From">Fabien &lt;[email protected]&gt;</framework:header>
+                    <framework:header name="Bcc">[email protected]</framework:header>
                     <framework:header name="X-Custom-Header">foobar</framework:header>
                 </framework:mailer>
             </framework:config>
@@ -578,8 +578,8 @@ and headers.
                     ->recipients(['[email protected]', '[email protected]'])
             ;
 
-            $mailer->header('from')->value('Fabien <fabien@example.com>');
-            $mailer->header('bcc')->value('[email protected]');
+            $mailer->header('From')->value('Fabien <fabien@example.com>');
+            $mailer->header('Bcc')->value('[email protected]');
             $mailer->header('X-Custom-Header')->value('foobar');
         };