Fix default headers merging

src/DefaultRestClientBuilder.php

@@ -13,6 +13,7 @@
 use Symfony\Component\Serializer\SerializerInterface;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
 
+use function is_string;
 use function strtolower;
 
 /**
@@ -60,7 +61,11 @@ public function defaultHeaders(array $headers): static
         foreach ($headers as $name => $values) {
             $name                          = strtolower($name);
             $this->defaultHeaders[$name] ??= [];
-            $this->defaultHeaders[$name]  += $values;
+            if (is_string($values)) {
+                $this->defaultHeaders[$name][] = $values;
+            } else {
+                $this->defaultHeaders[$name][] += $values;
+            }
         }
 
         return $this;

src/RestClientBundle.php

@@ -48,9 +48,13 @@ public function configure(DefinitionConfigurator $definition): void
                 ->scalarNode('base_uri')
                     ->info('The URI to resolve relative URLs, following rules in RFC 3985, section 2.')
                 ->end()
-                ->variableNode('default_headers')
+                ->arrayNode('default_headers')
                     ->info('Associative array of default headers: header => value(s).')
-                    ->defaultNull()
+                    ->defaultValue([])
+                    ->requiresAtLeastOneElement()
+                    ->useAttributeAsKey('name')
+                    ->normalizeKeys(false)
+                    ->variablePrototype()->end()
                 ->end()
             ->end()
         ->end();