NesHawk's MMC3 chip doesn't support disabling PRG RAM

[100thCoin]

Summary

With the MMC3 mapper, you can enable/disable the PRG RAM stored on the cartridge. When this PRG RAM is disabled, reading from the address $6000 through $7FFF range should return open bus, though currently NesHawk reads these addresses as if PRG RAM was always enabled.

Repro

I made a custom cartridge that runs the following ASM

   LDA #$00
   STA $A001
   LDA $7964 ; This address is part of the disabled PRG RAM

The expected result would leave the A register with a value of $79, due to open bus behavior.
Here's a screenshot of the tracelog, with the A register ending with a value of $00.