Does not handle non-alphanumerics properly

[Mak Nazečić-Andrlon]

irb(main):001:0> require 'damerau-levenshtein'
=> true
irb(main):002:0> DamerauLevenshtein.distance('abc', '-')
=> 0

[Dmitry Mozzherin]

It does not seem to be an issue with nonalphanumeric characters, but rather a case of #2 where one of the strings is a single character.