Corrige No contabiliza usarios con cierta skin

fixes #483
Merge branch 'master' of git://github.com/gamersmafia/gamersmafia into gh-483-conteo-skins
Unit tests para used_by_users_count
Merge branch 'master' of git://github.com/gamersmafia/gamersmafia into gh-483-conteo-skins
Mejora test para used_by_users_count
Merge pull request #492 from Draco351/gh-483-conteo-skins

Corrige No contabiliza usarios con cierta skin