Fixed Message::set_header_list as per review comments. Changed ClonablePtr to use clone() internally

include/cppkafka/header_list.h

@@ -159,11 +159,6 @@ public:
      */
     explicit operator bool() const;
     
-    /**
-     * \brief Indicates if this list owns the underlying handle or not.
-     */
-    bool is_owning() const;
-    
 private:
     struct NonOwningTag { };
     static void dummy_deleter(rd_kafka_headers_t*) {}
@@ -313,11 +308,6 @@ HeaderList<HeaderType>::operator bool() const {
     return static_cast<bool>(handle_);
 }
 
-template <typename HeaderType>
-bool HeaderList<HeaderType>::is_owning() const {
-    return handle_.get_deleter() != &dummy_deleter;
-}
-
 } //namespace cppkafka
 
 #endif //RD_KAFKA_HEADERS_SUPPORT_VERSION