FIX: file download URL is failng

[chrisbc]

example

query {
  node(id:"RmlsZTo2NDE5Mjc2") {
    ... on File {
      file_url
      file_name
      file_size
    }
  }
}

returns:

{
  "errors": [
    {
      "message": "'FileData' object has no attribute '_client'",
      "locations": [
        {
          "line": 4,
          "column": 7
        }
      ],
      "path": [
        "node",
        "file_url"
      ]
    }
  ],
  "data": {
    "node": {
      "file_url": null,
      "file_name": "openquake_hdf5_archive-T3BlbnF1YWtlSGF6YXJkVGFzazo2NjAwMjgx.zip",
      "file_size": 816143
    }
  }
}

[chrisbc]

obviously this must be missing an effective feature test!!