price set fields with $0 fee cannot be changed

to reproduce:

• create a price set with a text/numeric quantity field where the fee is $0
• create an event and assign the price set to the event
• register a person for the event and enter a number for the quantity field. save.
• edit the registration, select the option to change selections, and change the quantity. save.

you expect the quantity to reflect the change, but instead it remains the same. the issue only seems to affect the text/quantity field, and only when the fee rate is $0. I suspect that on save the form sees no change in the registration rate and so ignores the quantity field altogether.

reproducible on both the 4.6 and 4.7 demo sites.