Skills added by addSkill and addAllSkillls may not match

[syonekura]

Hi, fist of all, I found this library a week ago and it has been so useful and easy to use, so many thanks to the developers for their great job

I found this little bug when solving a VRPTW, in my case, I added the required skills to my services using addSkill, and then used addAllSkills on the vehicles, causing to get all the jobs unassigned, so i think addAllSkills should call toLowerCase as addSkill does, or call addSkill within the for loop:

public Builder addAllSkills(Collection<String> skills){
            for(String skill : skills) this.skills.add(skill);
            return this;
        }

public Builder addSkill(String skill){
            skills.add(skill.toLowerCase());
            return this;
        }

[oblonski]

Thanks @syonekura for your kind words and the bug detection.