Fix HP change in bear forms

Allow druids to keep the same currentHP/maxHP ratio when switching forms between humanoid and bear

This commit [e0a0426](http://github.com/mangos-zero/server-old/commit/e0a0426) is one of those pending backports from old mangos-zero concatenated in: https://github.com/cmangos/issues/wiki/classic_backporting-todo-zero-commits

I tested it and this is my:
> Shapeshifting into bear form, druids do not keep the same percentage of HP with current core revision. This commit fixes it by keeping constant currentHP/maxHP ratio.