Missed Required Field Returns Default Value of Membership after Overriding

I ran into this trivial issue when testing https://github.com/civicrm/civicrm-core/pull/8533 - as I was entering a credit card membership contribution for an amount greater than the configured membership price of $100, I forgot to enter the billing address - when the screen showing the missed required fields in red came back, the amount was reset to $100 instead of the $150 I had originally entered - trivial, but I can see where this could lead to some aggravated end users who may not catch the mistake at all.