Ensure space around binary exprs

[MarcusGrass]

Fixes https://github.com/rust-lang/rustfmt/issues/6059.

It just needed the same caveat for binary exprs.

It was already implemented for unaries so 2. .. 4. worked but not 4. / 3. .. 5. for example.

[ytmimi]

Also I think it would be helpful if we could work the original code snippet from https://github.com/rust-lang/rustfmt/issues/6059 into a test case.