Don't assume res._header exist

`res._header` is an internal implementation detail. Hence,
some implementations of the http server do not set this.
[node-spdy](https://github.com/indutny/node-spdy) is an example.

src/webjs.coffee

@@ -120,7 +120,7 @@ exports.GenericApp = class GenericApp
     log_request: (req, res, data) ->
         td = (new Date()) - req.start_date
         @log('info', req.method + ' ' + req.url + ' ' + td + 'ms ' +
-                (if res.finished then res._header.split('\r')[0].split(' ')[1] \
+                (if res.finished and res._header then res._header.split('\r')[0].split(' ')[1] \
                                   else '(unfinished)'))
         return data