Always initialize all plugins in registered_extensions()

[radarhere]

Resolves #6809

In registered_extensions(), if there are any EXTENSIONS present, then it doesn't call init().

Pillow/src/PIL/Image.py

Lines 3416 to 3423 in 7f6fe3c

	def registered_extensions():
	"""
	Returns a dictionary containing all file extensions belonging
	to registered plugins
	"""
	if not EXTENSION:
	init()
	return EXTENSION

This means that

from PIL import Image
print(Image.registered_extensions())

returns more extensions than

from PIL import Image
import PIL.PngImagePlugin
print(Image.registered_extensions())

does. This seems unexpected to the user.

The original logic came from #1968, which was merged in #2343.

This PR adjusts it to always call init(), to make sure that all plugins are initialised. This is not an expensive operation, as there is still a check for the _initialized value to see if it has already been done.

I've adjusted the test to remove the resetting of EXTENSION, which is no longer relevant to this function.