 
         // set headers
         session.defaultHeaders = defaultHeaders
+        if let headers = headers {
+            session.defaultHeaders.merge(headers) { (_, new) in new }
+        }
 
         // set callbacks
         session.onResults = success


        // set headers
        session.defaultHeaders = defaultHeaders

        // set callbacks
        session.onResults = success

        customizationID: String? = nil,
        learningOptOut: Bool? = nil,
        compress: Bool = true,
        headers: [String: String]? = nil,
        failure: ((Error) -> Void)? = nil,
        success: @escaping (SpeechRecognitionResults) -> Void)
    {
