[HttpClient] fix merge

Author: nicolas-grekas

HttpClientTrait.php

@@ -97,7 +97,7 @@ private static function prepareRequest(?string $method, ?string $url, array $opt
         }
 
         if (isset($options['body'])) {
-            if (\is_array($options['body']) && !isset($options['normalized_headers']['content-type'])) {
+            if (\is_array($options['body']) && (!isset($options['normalized_headers']['content-type'][0]) || !str_contains($options['normalized_headers']['content-type'][0], 'application/x-www-form-urlencoded'))) {
                 $options['normalized_headers']['content-type'] = ['Content-Type: application/x-www-form-urlencoded'];
             }