BUG/MINOR: http_htx: Fix searching headers by substring

Function __http_find_header is used to search headers by name using specified
matching method. Matching by substring returned unexpected results due to wrong
length of substring supplied to strnistr function.

Fixed also the boolean condition by inverting it, as we're interested in
headers that contains the substring.

This patch should be backported as far as 2.2
diff --git a/src/http_htx.c b/src/http_htx.c
index b27b083..9570b1e 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -201,7 +201,7 @@
 					goto next_blk;
 				break;
 			case HTTP_FIND_FL_MATCH_SUB:
-				if (strnistr(n.ptr, n.len, name.ptr, n.len) != NULL)
+				if (!strnistr(n.ptr, n.len, name.ptr, name.len))
 					goto next_blk;
 				break;
 			default: