Fix Viperized .Site.Params

git bisect identified 62dd1d4 as the breaking commit; when
github.com/spf13/viper was introduced, the Params field was always
empty.

Given a map in YAML in Viper, the return type is
`map[interface{}]interface{}`, _not_ `map[string]interface{}`, even if
`.SetDefault()` has been called with an item of
`map[string]interface{}{}` so the cast assertion on the `.Get("Params")`
always failed.