Restore field that was excluded from queries by generator

[rdavila]

I'm doing some extra requests because the price field was excluded in the customization of the RatePlanCharge class:

active_zuora/lib/active_zuora/generator.rb

Lines 198 to 200 in 00e75d8

	exclude_from_queries :overage_price, :included_units,
	:discount_amount, :discount_percentage, :rollover_balance, :price
	lazy_load :price

I don't have a volume/tiered charging model so in my case I don't have any reason to exclude that field