Verify that s_resp_bodybytes is indeed incremented the correct number of bytes...

Verify that s_resp_bodybytes is indeed incremented the correct number of bytes on partial content responses.

bin/varnishtest/tests/c00034.vtc

@@ -90,3 +90,5 @@ client c1 {
 	expect resp.status == 206
 	expect resp.bodylen == 100
 } -run
+
+varnish v1 -expect s_resp_bodybytes == 1040