Levenshtein distances Bug

[ibuda]

Found a bug in Levenshtein and WegihtedLevenshtein distances implementations.
In more details, the following methods give wrong #results:
Levenshtein.distance("abc", "a"): Expected: 2, Actual: 1
Levenshtein.distance("a", "abc") : Expected: 2, Actual: 1
WeightedLevenshtein.distance("a2c3e", "1b1d1", e): Expected: 8, Actual: WeightedLevenshtein.distance("a", "abc", e): Expected: 2, Actual: 1

This error is caused by a mistake in the implementation of Levenshtein distances.
Having studied the list of open issues, this bug causes #239 and #244.