getKeys doesn't delimit `String` values

[dbeacham]

getKeys outputs the same value for Data.Aeson.String "null" and Data.Aeson.Null.

λ> getKeys V1 (sl "nullString" ("null" :: String))
["nullString:null"]

λ> getKeys V1 (sl "nullValue" (Nothing :: Maybe ()))
["nullValue:null"]

Same for Boolean values.

I can write my own version but might be a good idea to provide a version that supplies a delimiter? Could get the current version to do that but would be a breaking change.

I'm happy to supply a PR if this seems reasonable (and I haven't missed something obvious that already does this).