Caesar cipher in Ruby

caesar_cipher

def caesar_cipher(input, shift)
  positions = input.unpack("C*")
  shifted_positions = positions.map do |pos|
    case pos
    when (65..90), (97..122)
      shifted = pos + (shift % 26)
      if (shifted > 90 && shifted < 97) || (shifted > 122)
        shifted = shifted - 26
      end
      pos = shifted
    else
      pos
    end
  end
  shifted_positions.pack("C*")
end

puts caesar_cipher("Fraq hf gur pbqr lbh hfrq gb qrpbqr guvf zrffntr", 13)