Unescape Document output path

Document#destination wasn't unescaped properly.

For example, when we have a document named '_langs/c#.md',
we expect its url to be '/langs/c#.html',
but it was actually '/langs/c%23.html'.

We now unecape URL at Document#destination like Post#destination and
Page#destination.
Merge pull request #2924 from nitoyon/unescape-document