revise overage price calculations

http://pm.tablexi.com/issues/79737
don't reassign actual_start_at, pass in as param to #calculate_overage

http://pm.tablexi.com/issues/79737
add spec for when usage is less than actual, but extends into overage
fix #over_reservation? to meet new definition

http://pm.tablexi.com/issues/79737
update description; update spec

http://pm.tablexi.com/issues/79737
refactor

http://pm.tablexi.com/issues/79737
check for nil in #sum_costs because #calculate_reservation can return it

http://pm.tablexi.com/issues/79737
Merge pull request #100 from tablexi/price_edits_79737

revise overage price calculations