Post#url was escaped using CGI.escape.
When file name contains a space character, its url points to
non-existing URL.
For example, when we have a post named '2014-01-02-foo bar.md',
we expect its url to be '/2014/01/02/foo%20bar.html',
but it was actually '/2014/01/02/foo+bar.html'.
We now define Jekyll::URL.escape_path and Jekyll::URL.unescape_path,
and use them to escape and unescape Post#url
nitoyon