remove datum param from valid keys methods

These methods will eventually look something like
```rb
def valid_connection_keys(params = {})
  datum = @data || {}
  middlewares = params[:middlewares] || datum[:middlewares] || []
  Excon::VALID_CONNECTION_KEYS | middlewares.map {|m| m.valid_connection_keys }.flatten
end
```
Although, I'm still considering whether or not middlewares should be able to make keys invalid as well.