minor symfony#30681 [Mime] Removed unnecessary strtolower function call (scuben)

This PR was merged into the 4.3-dev branch.

Discussion
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[Mime] Removed unnecessary strtolower function call

| Q             | A
| ------------- | ---
| Branch?       | master <!-- see below -->
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| License       | MIT

Commits
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9419887 [Mime] Removed unnecessary strtolower

Author: fabpot

src/Symfony/Component/Mime/Header/Headers.php

@@ -168,7 +168,7 @@ public function add(HeaderInterface $header)
     public function get(string $name): ?HeaderInterface
     {
         $name = strtolower($name);
-        if (!isset($this->headers[strtolower($name)])) {
+        if (!isset($this->headers[$name])) {
             return null;
         }