fix(plugin-express): Ensure HTTP status is 500 when crashing

The wrong property was being used in attempt to set the HTTP status code. This fixes it for Express
and Connect (connect doesn't have the method res.status()).

Fixes #510.

packages/plugin-express/src/express.js

@@ -37,7 +37,7 @@ module.exports = {
           req.bugsnag.config.onUncaughtException(err, report, client._logger)
         })
         if (!res.headersSent) {
-          res.status = 500
+          res.statusCode = 500
           res.end('Internal server error')
         }
       })