[Mailer] Fixed mandrill api header structure

Author: wulff

src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillApiTransportTest.php

@@ -63,7 +63,7 @@ public function testCustomHeader()
         $this->assertArrayHasKey('message', $payload);
         $this->assertArrayHasKey('headers', $payload['message']);
         $this->assertCount(1, $payload['message']['headers']);
-        $this->assertEquals('foo: bar', $payload['message']['headers'][0]);
+        $this->assertEquals('bar', $payload['message']['headers']['foo']);
     }
 
     public function testSend()

src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php

@@ -111,7 +111,7 @@ private function getPayload(Email $email, Envelope $envelope): array
                 continue;
             }
 
-            $payload['message']['headers'][] = $name.': '.$header->getBodyAsString();
+            $payload['message']['headers'][$name] = $header->getBodyAsString();
         }
 
         return $payload;