minor #15191 [Testing] Every custom header in functional testing client must have HTTP_ prefix (Ferror)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead. Discussion ---------- [Testing] Every custom header in functional testing client must have HTTP_ prefix <!-- If your pull request fixes a BUG, use the oldest maintained branch that contains the bug (see https://symfony.com/releases for the list of maintained branches). If your pull request documents a NEW FEATURE, use the same Symfony branch where the feature was introduced (and `5.x` for features of unreleased versions). --> Due to symfony/symfony#5074 issue, I would propose to add a simple caution block with information, that the `HTTP_` prefix is mandatory - it will save some hours for others. Commits ------- 8e5d9bc Every custom header in testing client must have HTTP prefix

Author: javiereguiluz

testing.rst

@@ -1029,6 +1029,10 @@ You can also override HTTP headers on a per request basis::
  'HTTP_USER_AGENT' => 'MySuperBrowser/1.0',
  ]);
 
+.. caution::
+
+ Every custom header must have `HTTP_` prefix.
+
 .. tip::
 
  The test client is available as a service in the container in the ``test``