BUG/MINOR: h2: reject response pseudo-headers from requests

At the moment there's only ":status". Let's block it early when parsing
the request. Otherwise it would be blocked by the HTTP/1 code anyway.
This silences another h2spec issue.

To backport to 1.8.
diff --git a/src/h2.c b/src/h2.c
index 41565c0..83ef043 100644
--- a/src/h2.c
+++ b/src/h2.c
@@ -212,6 +212,10 @@
 		*(out++) = '\n';
 	}
 
+	/* RFC7540#8.1.2.1 mandates to reject response pseudo-headers (:status) */
+	if (fields & H2_PHDR_FND_STAT)
+		goto fail;
+
 	/* Let's dump the request now if not yet emitted. */
 	if (!(fields & H2_PHDR_FND_NONE)) {
 		ret = h2_prepare_h1_reqline(fields, phdr_val, &out, out_end);