Update meta charset

This pull request attends the todo-comment according the update of documents meta charset tag.

If the charset is changed, the charset meta-tag of the document is slected and updated (if existing).

An improvement could be an overload `charset()` method, where the caller can choose if the meta tag will be updated too.

A test is included.
What about the other forms of this? `meta name=charset value=`, and `meta htt-equiv contenttype`? There's code in DataUtils for that, should be able to reuse.