Saving still WebP image does not use method argument #4042

What did you do?

Add the method argument (as documented in the handbook) with 0 as value to the Image.save() function with webp as format.

What did you expect to happen?

Pillow/WebP encoding the image with that method.

What actually happened?

The image always got encoded with the default method.

What are your OS, Python and Pillow versions?

• OS: Debian testing (bullseye)
• Python: 3.7.4
• Pillow: 6.1.0 (from pypi)

Used image: https://upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Python-logo-notext.svg/512px-Python-logo-notext.svg.png (as image.png)

from PIL import Image
from io import BytesIO

image = Image.open('image.png')

buffer = BytesIO()
image.save(buffer, format='webp', method=0)
print(buffer.getbuffer().nbytes) # 9212

buffer = BytesIO()
image.save(buffer, format='webp', method=6)
print(buffer.getbuffer().nbytes) # 9212

Both of these have the exact same size, which should not be the case given the different method.
Example of different sizes (encoding with cwebp tool):

$ cwebp -m 0 image.png 2>&1 | grep Output
Output:    11396 bytes Y-U-V-All-PSNR 52.79 45.11 45.20   48.64 dB
$ cwebp -m 6 image.png 2>&1 | grep Output
Output:    8408 bytes Y-U-V-All-PSNR 53.12 44.73 44.82   48.43 dB

Encoding as animation, however, uses the set method for encoding, as you can see from the different sizes:

# (continuation from above code snippet)

buffer = BytesIO()
image.save(buffer, format='webp', save_all=True, append_images=[image], method=0)
print(buffer.getbuffer().nbytes) # 12310

buffer = BytesIO()
image.save(buffer, format='webp', save_all=True, append_images=[image], method=6)
print(buffer.getbuffer().nbytes) # 9220

This pull request might be related: #1263