(maint) Improve Puppet::Util::Docs.scrub's handling of one-liners

One-liner handling wasn't behaving as expected. Sometimes one-liners are
buffered with extra newlines before and/or after, and in these cases, scrub
wasn't catching the leading whitespace.

This commit makes it behave as intended, and adds a test showing the intended
behavior.
Merge pull request #2361 from nfagerlund/maint_scrub_improvement

(maint) Improve Puppet::Util::Docs.scrub's handling of one-liners