Handle webgistix multiple tracking numbers for one invoice

### Problem
Sometimes a certain invoice at webgistix is split up in two different shipping methods. This results in two or more `<Shipment>` elements with the same invoice number, but different tracking numbers. Currently this is not handled properly by the gem, because everything is put in a hash with the invoice number as key. Thus overwriting the previous tracking numbers by the last one.

### Solution
Multiple tracking numbers for the same key was already supported, but unfortunately not used. I've made it so that tracking number items are properly grouped. I've added a fixture + test case for it to confirm the behaviour.

@pickle27 @costford @csaunders can any of you review this when you have the time?
/cc @jduff @costford @pickle27