Update http_client.rst

Integrating the findings of symfony/symfony#38082 (comment)

Author: ThomasLandauer

http_client.rst

@@ -642,6 +642,11 @@ when the streams are large)::
         'body' => $formData->bodyToString(),
     ]);
 
+If you need to add a custom HTTP header to the upload, you can do::
+
+    $headers = $formData->getPreparedHeaders()->toArray();
+    $headers[] = 'X-Foo: bar';
+
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