OData Service Document format problems with PowerBI

I'm attempting to add OData support to an existing node+express app. When I try to connect PowerBI with an OData Feed to my Service Document, the error message is "The format 'application/atomsvc+xml' is not supported". (I've also tried "text/plain", "application/xml", and "application/atom+xml" with similar error messages).

I'm constructing the service document based on http://docs.oasis-open.org/odata/odata-atom-format/v4.0/cs02/odata-atom-format-v4.0-cs02.html#_Service_Document.

<service xmlns="http://www.w3.org/2007/app" xmlns:atom="http://www.w3.org/2005/Atom" xmlns:metadata="http://docs.oasis-open.org/odata/ns/metadata" xml:base="https://<my-server>/api/odata/odata-xml.svc" metadata:context="$metadata">
<workspace>
  <atom:title>Default</atom:title>
    <collection href="groups">
      <atom:title>groups</atom:title>
    </collection>
    <collection href="users">
      <atom:title>users</atom:title>
    </collection>
  </workspace>
</service>

I've tried serving this as an attachment, inline file, and as text with DataServiceVersion set to '1.0' to match the sample at http://services.odata.org/northwind/northwind.svc, and can't see what's different. The OData-Version header is set to '4.0'.

I've temporarily disabled authentication and authorization on this endpoint to make sure it wasn't interfering with getting a basic connection going.

I also have defined the /api/odata/$metadata and a JSON service document at the root /api/odata but PowerBI isn't hitting those endpoints. From logging the headers that it sends, it seems like it should support all of these formats:

{
  maxdataserviceversion: '3.0',
  'odata-maxversion': '4.0',
  accept: 'application/json;odata.metadata=minimal;q=1.0,application/json;odata=minimalmetadata;q=0.9,application/atomsvc+xml;q=0.8,application/atom+xml;q=0.8,application/xml;q=0.7,text/plain;q=0.7',
  'accept-encoding': 'gzip, deflate',
  host: 'oas-testlab.spanoctane.com'
}

I haven't found what formats are supported, just a bunch that aren't. Has anyone had any success building a node+express app that connects to PowerBI? OR do you notice anything blatantly obvious that's missing or wrong in what I've tried?