support multiple product rate plans in subscribe requests
adding method for backward compatibility of single product_rate_plan in subreq
product_rate_plan= method shouldn't be protected
DRYing object xml generation methods
specs to cover modifications to validation methods and turning product_rate_plan into product_rate_plans
Revert "Updated @nirvdrum spec to support new return values"

This reverts commit fd511948c9d0b60bc0de277760be05217faab85e.
Revert "Allow passing just the product_rate_plan_id rather than a fully materialized object when creating a SubscribeRequest."

This reverts commit eb669038d749b71a162ff7f694893f8e5fbfe3ef.
Merge remote-tracking branch 'jmoline/multiple_rate_plans' into 1.0

Support multiple rate plans in a subscription request.

Conflicts:
	lib/zuora/objects/subscribe_request.rb