The “newline” name seems as good as any. The patch looks good in general. I left a few comments.

I agree with not bothering for the UU encoder. That encoding has an embedded line length byte at the start of each line, so it makes less sense to omit newlines. Base-64 is used in places such as URLs where multiline output is not relevant, but I don’t know of a similar case for the UU encoding.

If I was doing it, I wouldn’t bother with a test case for keyword-only-ness, but suit yourself.