Add the ability to handle no credentials.

Signature v4 requests were trying to create authorisation strings when no credentials were present.  This change adds the v2 lines that handle this.

hfile_s3.c

@@ -1058,6 +1058,10 @@ static int v4_auth_header_callback(void *ctx, char ***hdrs) {
         return -1;
     }
 
+    if (!ad->id.l || !ad->secret.l) {
+        return copy_auth_headers(ad, hdrs);
+    }
+
     hash_string("", 0, content_hash); // empty hash
 
     ad->canonical_query_string.l = 0;