Consider adding end-to-end tests for creating assets to Smokey

[floehopper]

Currently it looks as if Smokey only makes requests for existing assets:

• https://github.com/alphagov/smokey/blob/master/features/asset_manager.feature
• https://github.com/alphagov/smokey/blob/master/features/assets.feature

I think it might make sense to add some tests which create a Mainstream and a Whitehall asset. Additionally it might make sense to do some more detailed checking of the response headers.