Fixed issue #396 and added some tests to catch it

include/spdlog/async_logger.h

@@ -63,6 +63,11 @@ public:
     //Wait for the queue to be empty, and flush synchronously
     //Warning: this can potentialy last forever as we wait it to complete
     void flush() override;
+
+	// Error handler
+	virtual void set_error_handler(log_err_handler) override;
+	virtual log_err_handler error_handler() override;
+
 protected:
     void _sink_it(details::log_msg& msg) override;
     void _set_formatter(spdlog::formatter_ptr msg_formatter) override;