Tests: fixed HTTP/2 PUSH_PROMISE frame parsing.

lib/Test/Nginx/HTTP2.pm

diff --git a/lib/Test/Nginx/HTTP2.pm b/lib/Test/Nginx/HTTP2.pm
index 30028c1..0cf257b 100644
--- a/lib/Test/Nginx/HTTP2.pm
+++ b/lib/Test/Nginx/HTTP2.pm
@@ -404,6 +404,7 @@
 
 sub push_promise {
 	my ($ctx, $buf, $len, $flags) = @_;
+	$len -= 4;
 
 	{ promised => unpack("N", $buf),
 	  headers => hunpack($ctx, substr($buf, 4, $len), $len) };