hook_civicrm_membershipTypeValues doesn't affect values on form submit

hook_civicrm_membershipTypeValues() performs unexpectedly.

I thought it would let me set the minimum_fee to 0 for some membership types, and it does work okay when loading the membership contribution page, but on submit, I still get the error: "The Membership you have selected requires a minimum contribution of $ 300.00"

Turns out (as lobo pointed out on IRC), formRule() is not using the hook-modified values. Lobo suggested caching the values of the function buildMembershipTypeValues() in the form, and then letting formRule() use that cached data.

This patch makes those changes.