Feature/xero handle data lists

The issue

The DF_Xero library used array2xml to produce the XML for Xero objects. This chokes on data objects that contain lists of multiple sub-objects. Examples include the <Addresses> and <Phones> elements when creating a new Xero contact.

With this change, a flat array (no keys, or numeric keys) can be supplied as part of a data object. The parent element name will be singularized to provide the XML subelement names.

e.g.

[
    'Addresses' => [
        {address data array},
        {address data array}
    ]
]

==>

<Addresses><Address>{address data}</Address><Address>{address data</Address></Addresses>

What I did

• Replaced the existing array->xml dependency with some 'borrowed' methods that allow for data elements that contain lists of subelements.
  • Methods came from a recommended third-party library we have used in Art Money: https://github.com/calcinai/xero-php

Implications

None

Setup

None

How to test

• Code review.
• Can be tested with commit 62c34e40a5c06ad29c4242630c98d059072e3537 in the National Weighing Xero integration project, but might be easier to OTS it at first opportunity.