Fix failure on handling the path containing non-uri chars

In `DraftArticle#path_part`, `Addressable::Template#extract`
returns `nil` when `path` has characters not allowed for
URI (RFC3986) so that `DraftArticle#slug` fails for undefined `[]`.
Normalizing (percent-encoding) `path` string before `extract`
prevents its failure.

Characters outside the allowed set are Greek, Cyrillic or Japanese for example.
see http://www.ietf.org/rfc/rfc3986.txt