lib/Message/HeaderConverter.php
public static function toPsrHeaders(array $headers): array
    {
        $psr = [];
        foreach ($headers as $header) {
            list($key, $value) = explode(':', $header, 2);
            $psr[trim($key)][] = trim($value);
        }

        return $psr;
    }
public static function toPsrHeaders(array $headers): array
    {
        $psr = [];
        foreach ($headers as $header) {
            list($key, $value) = explode(':', $header, 2);
            $psr[trim($key)][] = trim($value);
        }

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