Unescaped `=` is not accepted as a valid character of `SimpleCommand`

**Describe the bug**
`=` is not recognized when parsing commands.

For details see https://bugs.launchpad.net/ubuntu/+source/rust-sudo-rs/+bug/2123859

In our parser, it's required to escape `=` to get it accepted, that is probably based on a reading of a manpage (or a false assumption).