feat #500 String.dashedToCamel|camelToDashed
fix #499 conform standards regarding dataset in Html.buildAttributeList

1) Added check for invalid keys passed to `dataset` in
utils.Html.buildAttributeList.

So far it only logs warning, to be switched in backward-incompatible
release.

2) Add utils.String.dashedToCamel and .camelToDashed

Close #500.