Support `name` containing whitespace

[MarkLodato]

The current implementation disallows whitespace in the subject name because it uses strings.Fields. The spec does not disallow whitespace, and spaces in filenames are not uncommon, so it would be best if the implementation allowed that.

To fix, using strings.Cut (or strings.SplitN or a regexp) would avoid the issue.

[ianlewis]

Yeah, that was an oversight. I didn't realize that sha256sum doesn't escape spaces in the name.