RAM (0x00 - 0x7FF) is not mirrored (through 0x800 - 0x1FFF)

[mmcdole]

It doesn't look like RAM access is mirrored per this diagram:
http://en.wikibooks.org/wiki/NES_Programming/Memory_Map

I believe a simple fix would be to bitwise & the address with 0x7FFF which should be equivalent to modulus 0x800.

func (m Memory) Read(a uint16) (Word, error) {
    switch {
    case a >= 0x2008 && a < 0x4000:
        offset := a % 0x8
        return ppu.RegRead(int(0x2000 + offset))
    case a <= 0x2007 && a >= 0x2000:
        return ppu.RegRead(int(a))
    case a == 0x4016:
        return Pads[0].Read(), nil
    case a == 0x4017:
        return Pads[1].Read(), nil
    case a&0xF000 == 0x4000:
        return apu.RegRead(int(a))
    case a >= 0x8000 && a <= 0xFFFF:
        return rom.Read(int(a)), nil
    case a >= 0x5100 && a <= 0x6000:
        if _, ok := rom.(*Mmc5); ok {
            // MMC5 register handling
            return rom.Read(int(a)), nil
        }
    }

    return m[a & 0x7FFF], nil // <--- Mirroring handled here
}