Fix CanadaPostPWS#find_rates not sending dimensions

[maartenvg]

Problem

Currently the common interface for find_rates is def find_rates(origin, destination, packages, options = {}). This is the case for all carriers, except CanadaPost and CanadaPostPWS. They use def find_rates(origin, destination, line_items = [], options = {}, package = nil, services = []) where line_items supposedly is an array of PackageItems. This then bubbles down via build_rate_request to parcel_node. There it takes package for the dimensions and package or line items for the weight.

Because the interface for CanadaPostPWS deviates from the others, any user that wants to send a consistent set of params to the carriers ends up sending packages to something that is called line_items on canada post, and leaves package nil. Which then results in a parcel_node with no package and only line items. This results in a parcel XML node that only contains the weight (from the sum of the line items) and no dimensions.

Solution

In this PR I'm changing a few things at once

• Use same method signature as all the other find_rates endpoints: i.e. got rid of line_items and replaced it with packages. I left the services array for now.
• Since CP only supports one package per shipment, I then grab the first package to pass into build_rates_request
• With only one package and no line items parcel_node changes a bit to always get its information from that one package.

[maartenvg]

Closing since this is going to be solved elsewhere (outside of this repo)