Use mode as default for C getpalette raw mode

[radarhere]

In C's getpalette(), the default for the raw mode argument is "RGB"

Pillow/src/_imaging.c

Lines 1185 to 1193 in 9e282f5

	_getpalette(ImagingObject *self, PyObject *args) {
	PyObject *palette;
	int palettesize;
	int bits;
	ImagingShuffler pack;
	char *mode_name = "RGB";
	char *rawmode_name = "RGB";
	if (!PyArg_ParseTuple(args, "|ss", &mode_name, &rawmode_name)) {

However, I don't find this default is used anywhere. Instead, I've found four calls like

Pillow/src/PIL/Image.py

Line 1349 in 9e282f5

palette_data = im.im.getpalette(mode, mode)[: colors * len(mode)]

I think it makes more sense to use the mode as the default for the raw mode.