use sub instead of Type::Tiny isa check for Product->quantity

Workaround for #28 until time is available to investigate fully.

lib/Interchange6/Cart/Product.pm

@@ -149,8 +149,16 @@ than zero. Default for quantity is 1.
 =cut
 
 has quantity => (
-    is      => 'ro',
-    isa     => PositiveInt,
+    is => 'ro',
+    # The following isa check is failing on some smokers for an (as yet)
+    # unknown reason. See:
+    # https://github.com/interchange/Interchange6/issues/28
+    # so for now we use a sub instead of the Type::Tiny type
+    #isa     => PositiveInt,
+    isa => sub {
+        die "quantity must be a positive integer"
+          unless ( $_[0] && $_[0] =~ /^\d+$/ )
+    },
     default => 1,
     writer  => 'set_quantity',
 );