decodeURI when use this.path as key to fetch value from files object

The path of some resources are non-standard,may contains Chinese
characters or something else,these characters are encoded,
static_cache can't recgonize them well.
use safe decode function ,for robust

safe decode function from
https://github.com/alexmingoia/koa-router/blob/master/lib/route.js#L202
Merge pull request #26 from codepiano/decodeURI

decodeURI when use this.path as key to fetch value from files object