bugfix(headers): default headers should overwrite built in headers

CHANGELOG.md

+## [1.2.1] - 2020-12-08
+### Fixed
+- default headers should overwrite build in headers
+
 ## [1.2.0] - 2019-06-12
 ### Fixed
 - CacheResolver in cached client
+
 ## [1.1.0] - 2019-05-30
 ### Added
 - Timeout in api client options interface

src/Client/ClientImplementation.php

@@ -151,7 +151,7 @@ class ClientImplementation implements Client, LoggerAwareInterface
             'Accept-Encoding' => '*',
             'Content-Type' => $this->serializer->getMime()
         );
-        $headers = array_merge($this->defaultRequestHeaders, $headers);
+        $headers = array_merge($headers, $this->defaultRequestHeaders);
         return array_merge($headers, $request->getHeaders());
     }