[openstreetmap/openstreetmap-website] 502 Proxy error on changeset download API (#2594)

[Tobias Zwick]

When repeatedly querying the changeset download API, rarely but sometimes the following HTTP error 502 is returned:

![image](https://user-images.githubusercontent.com/4661658/79670238-d0349680-81c1-11ea-83ff-cb2e23bc1b4d.png)

With the reason given: **DNS lookup failure for: rails3**

### How to reproduce

I could reproduce it by opening an url like https://api.openstreetmap.org/api/0.6/changeset/80149421/download in the browser and repeatedly reload the page, lots of times, until the error appears.

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