There is already a TaggedEC2Object.add_tag() method but it only supports
adding one tag at a time. The CreateTags api supports adding multiple tags
at once. So, when you want to add multiple tags you needlessly make multiple
round-trips to EC2 service. The add_tags method helps in avoiding that.

The documentation says that there is a distinction between a
value of '' and a value of None. The API docs say the same
thing. But the code was not following the docs:
    if value:
returns False for both None and ''.