"optimize" default when saving GIF images

[radarhere]

https://pillow.readthedocs.io/en/latest/handbook/image-file-formats.html#gif-saving states that when saving GIF, you can specify "optimize", and that it is off by default.

optimize
If present and true, attempt to compress the palette by eliminating unused colors. This is only useful if the palette can be compressed to the next smaller power of 2 elements.

This doesn't line up with the reality - it is only off by default if "palette" is provided in im.encoderinfo or im.info.

Pillow/src/PIL/GifImagePlugin.py

Lines 653 to 659 in 04a4d54

	def _save(im, fp, filename, save_all=False):
	# header
	if "palette" in im.encoderinfo or "palette" in im.info:
	palette = im.encoderinfo.get("palette", im.info.get("palette"))
	else:
	palette = None
	im.encoderinfo["optimize"] = im.encoderinfo.get("optimize", True)

This PR corrects the documentation, and also simplifies the code by using setdefault().