Fixed addition of a tag with a different case.

Hi,

There is an issue when adding a tag to an item, then adding the same tag with a different case. Currently, `Table_Tag::findOrNew()` searches for a collate utf8_bin one, but `AbstractRecord::fieldIsUnique()` searches for a case-insensitive one. This is a solution to this issue, but another one can be done if tag should be sensitive.

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management