fix: merge headers rather than replacing them (#18)

Co-authored-by: Gregory Haddow <[email protected]>

src/Core/Responses/Concerns/IsResponsable.php

@@ -154,11 +154,12 @@ public function withHeader(string $name, string $value = null): self
      * Set response headers.
      *
      * @param array $headers
+     * @param bool $merge
      * @return $this
      */
     public function withHeaders(array $headers): self
     {
-        $this->headers = $headers;
+        $this->headers = array_merge($this->headers, $headers);
 
         return $this;
     }