Newly-required field causes pricing export to fail

In July 2014, effective-start-date became a required parameter for the Sears pricing API. After adding that parameter to our export, I saw it fail with the following error: 

> Invalid XML: Schema Validation failed at Line number 2 cvc-complex-type.2.4.d: Invalid content was found starting with element 'effective-start-date'. No child element is expected at this point.

Googling suggests there's a disconnect between the XML structure and the XSD against which it is validated. The recommended solution is to ensure the parameter has an unbounded maxOccurs. It is not possible to alter the XSD file, which can be viewed here: 

https://seller.marketplace.sears.com/SellerPortal/s/schema/rest/pricing/import/v3/dss-seller-pricing.xsd

PR makes `effective-start-date` optional for the pricing API, so that implementers can choose to exclude it until such time as the bug is resolved.