BUG/MINOR: h2: the TE header if present may only contain trailers

h2spec reports this issue which has no side effect for now, but is
better cleared.

To backport to 1.8.
diff --git a/src/h2.c b/src/h2.c
index 3c580d1..41565c0 100644
--- a/src/h2.c
+++ b/src/h2.c
@@ -179,6 +179,9 @@
 		if (isteq(list[idx].n, ist("host")))
 			fields |= H2_PHDR_FND_HOST;
 
+		if (isteq(list[idx].n, ist("te")) && !isteq(list[idx].v, ist("trailers")))
+			goto fail;
+
 		/* cookie requires special processing at the end */
 		if (isteq(list[idx].n, ist("cookie"))) {
 			list[idx].n.len = -1;