BUG/MINOR: http: fix incorrect header value offset in replace-hdr/replace-value

The two http-req/http-resp actions "replace-hdr" and "replace-value" were
expecting exactly one space after the colon, which is wrong. It was causing
the first char not to be seen/modified when no space was present, and empty
headers not to be modified either. Instead of using name->len+2, we must use
ctx->val which points to the first character of the value even if there is
no value.

This fix must be backported into 1.5.
diff --git a/src/proto_http.c b/src/proto_http.c
index 3c87478..b8f5520 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -3250,7 +3250,7 @@
 	while (http_find_full_header2(name, name_len, buf, idx, ctx)) {
 		struct hdr_idx_elem *hdr = idx->v + ctx->idx;
 		int delta;
-		char* val = (char*)ctx->line + name_len + 2;
+		char* val = (char*)ctx->line + ctx->val;
 		char* val_end = (char*)ctx->line + hdr->len;
 		char* reg_dst_buf;
 		uint reg_dst_buf_size;