Align XML structure of models with sequence defined in XSDs

PR alters the toArray() methods of three models: Product, Inventory, and Pricing. It does two things: 

1. Distinguishes required from optional parameters by putting them in separate arrays named $required and $optional.
2. Adjusts the structure of the array returned so that the sequence and arrangement of XML elements matches what is laid out in the XSDs.
Changed format of this element because the XSD requires a date here, not a timestamp.