Should AssetPresenter#as_json be using Plek#public_asset_host vs Plek#asset_root

[floehopper]

See this line of code:

asset-manager/app/presenters/asset_presenter.rb

Line 15 in a12a1a8

file_url: URI.join(Plek.new.asset_root, Addressable::URI.encode(@asset.public_url_path)).to_s,

I think Plek#public_asset_host will return assets.publishing.service.gov.uk in production whereas I think Plek#asset_root will return assets-origin.publishing.service.gov.uk. I haven't thought about it very carefully, but I think we want the former rather than the latter.