GET /multi/:id doesn't seem to work

Hi!

I'm in the process of building something with wzrd.in (great service!), but retrieving multi-bundles doesn't seem to work.

First, I posted to /multi:

```json
{
"dependencies": {
"react": "15.5.4",
"react-dom": "15.5.4"
}
}
```

That worked, and I got the response with the two packages and the bundles. I also found the location in the headers. But when I try to retrieve the multi-bundle, all I get is "Bundle not found, have you tried POSTing to /multi".

Is this broken, or am I doing something wrong?

Here's the url I'm trying to fetch: `https://wzrd.in/multi/b4e028f7b3be57e6572282cb26a89602`

Reproduce the POST to /multi with the dependency payload, then request the returned /multi/:id URL and trace the endpoint handling both operations. Done means the bundle created by the shown request can be retrieved from its returned location, with the reported failure reproduced or explained.

Written by the indexing model from the issue text.