Tests: allow to set empty HTTP/2 connection preface.

lib/Test/Nginx/HTTP2.pm

diff --git a/lib/Test/Nginx/HTTP2.pm b/lib/Test/Nginx/HTTP2.pm
index 0cf257b..24900f6 100644
--- a/lib/Test/Nginx/HTTP2.pm
+++ b/lib/Test/Nginx/HTTP2.pm
@@ -36,8 +36,8 @@
 	my ($port, %extra) = @_;
 
 	my $s = $extra{socket} || new_socket($port, %extra);
-	my $preface = $extra{preface}
-		|| 'PRI * HTTP/2.0' . CRLF . CRLF . 'SM' . CRLF . CRLF;
+	my $preface = defined $extra{preface} ? $extra{preface}
+		: 'PRI * HTTP/2.0' . CRLF . CRLF . 'SM' . CRLF . CRLF;
 
 	if ($extra{proxy}) {
 		raw_write($s, $extra{proxy});