Java: Fixed GameItem for unequippable items
Java: Fixed GameItem for unequippable items

Ruby and PHP already handled that case, but the documentation has been
adapted to make this more clear.

Closes #121

koraktor/steam-condenser-ruby,jpalumickas/steam-condenser-ruby,Arie/steam-condenser-ruby,bfosberry/steam-condenser-ruby,kickback/steam-condenser-ruby,hcool/steam-condenser-ruby