IsAppUrl filter & curl

https://github.com/Meteor-Community-Packages/meteor-fast-render/blob/0a3e50c69e276ab7e2c988c113a2e5fa3a1df002/lib/server/utils.js#L29

IsAppUrl makes SSR feature impossible with crawlers like curl which have accept header fixed to `*/*` (all)

Maybe it should be replaced by :
```es
return /html|\*\/\*/.test(req.headers.accept);
```

Start at lib/server/utils.js line 29 and inspect how IsAppUrl evaluates the request Accept header. Reproduce the behavior with a crawler such as curl using its */* header, then verify that HTML and */* requests are treated as app URLs without breaking other cases.

Written by the indexing model from the issue text.