[fix] Do not add the same distinguished name to a DN-syntax-based attribute more than once

If such DN is already in the attribute, do not add it again but also do
not complain about it.
[fix] The Attribute::remove() method was not removing items passed as values
[fix] Fix accessing values from the end in Attribute::value() using negative indexes
Update changelog for 0.2.2
Merge branch 'release/0.2.2' into develop