allow extra fields at top level

Some fields are added from the front end rather than an author.json
file. Since we aren't updating those, we shouldn't count them as
different.

lib/MetaCPAN/Util.pm

@@ -163,10 +163,11 @@ sub single_valued_arrayref_to_scalar {
 }
 
 sub diff_struct {
-    my (@queue) = [ @_, '' ];
+    my ( $old_root, $new_root, $allow_extra ) = @_;
+    my (@queue) = [ $old_root, $new_root, '', $allow_extra ];
 
     while ( my $check = shift @queue ) {
-        my ( $old, $new, $path ) = @$check;
+        my ( $old, $new, $path, $allow_extra ) = @$check;
         if ( !defined $new ) {
             return [ $path, $old, $new ]
                 if defined $old;
@@ -184,7 +185,8 @@ sub diff_struct {
         }
         elsif ( is_plain_hashref($new) ) {
             return [ $path, $old, $new ]
-                if !is_plain_hashref($old) || keys %$new != keys %$old;
+                if !is_plain_hashref($old)
+                || !$allow_extra && keys %$new != keys %$old;
             push @queue, map [ $old->{$_}, $new->{$_}, "$path/$_" ],
                 keys %$new;
         }