Hotfix imp[].ext.context.data.adslot issue

[rpanchyk]

This PR fixes First Party Data applying for imp[].ext.context.data.
Especially, mapping imp[].ext.context.data.adslot to imp[].ext.rp.target.dfp_ad_unit_code.

Before, to apply FPD we had to specify bidders explicitly:

  "ext": {
    "prebid": {
      "data": {
        "bidders": [ "rubicon" ]
      }
  }

Take into account, this is just a hotfix. After prebid/prebid-server#879 reworked - changes may be removed.