lib/Message/HeaderConverter.php
public static function toBuzzHeaders(array $headers): array
    {
        $buzz = [];

        foreach ($headers as $key => $values) {
            if (!\is_array($values)) {
                $buzz[] = sprintf('%s: %s', $key, $values);
            } else {
                foreach ($values as $value) {
                    $buzz[] = sprintf('%s: %s', $key, $value);
                }
            }
        }

        return $buzz;
    }
public static function toBuzzHeaders(array $headers): array
    {
        $buzz = [];

        foreach ($headers as $key => $values) {
            if (!\is_array($values)) {
                $buzz[] = sprintf('%s: %s', $key, $values);
            } else {
                foreach ($values as $value) {
                    $buzz[] = sprintf('%s: %s', $key, $value);
                }
            }
        }

        return $buzz;
    }
Convert from Buzz style headers to PSR style.