bug using 'scalar' inappropriately in cart route

[SysPete]

@murwiz added this to #25. New issue created to track this.

I think there's a bug in Interchange6::Routes::Cart:

    if (scalar($input)) {

I think you meant "ref($input)". I'm attempting to rewrite this code for my own purposes, then maybe you can look it over.

[SysPete]

Since cart route only accepts a single sku param right now (or at least can only handle a single one) this is currently harmless. Keeping this bug open until decision is made on how we will handle post with multiple products (see #25).