Fix `serveFiles` to work w/ dynamic swaggerDoc

[chgeo]

serveFiles did not respect on-the-fly changes in a swagger doc, but rendered the old content.

The fix now also enables this pattern, where serveFiles doesn't need the swaggerDoc upfront (which is desired in dynamic cases):

app.use('/api-docs-dynamic', function(req, res, next){
  swaggerDocument.info.description = `Hello ${count}!`;
  req.swaggerDoc = swaggerDocument;
  next();
}, swaggerUi.serveFiles(), swaggerUi.setup());
//                     ^^^

Also fixes #318 which stumbled over the same issue, but draw the wrong conclusion that the documentation would be wrong.

[chgeo]

@scottie1984 Can you review?