add missing test to sum-of-multiples exercise
add missing test to sum-of-multiples exercise

tommyschaefer/xruby,rashmiagar/exercism,NeimadTL/ruby,kotp/xruby,rashmiagar/exercism,gchan/xruby,exercism/xruby,NeimadTL/ruby,Insti/exercism-ruby,NeimadTL/ruby,remcopeereboom/xruby,exercism/xruby,Insti/xruby,Insti/exercism-ruby,Insti/exercism-ruby,Insti/xruby,kotp/xruby,gchan/xruby,remcopeereboom/xruby,tommyschaefer/xruby