typo: subclassRe_s_ponsibility
Fixes #479. {String,Array}>>at:ifPresent:ifAbsent:

It was subclass responsibility, but was not implemented.
at:ifPresent: was implemented using it so it was failing as well.
Merge pull request #480 from herby/gh-479

Implementing subclass responsibilities for at:ifPresent:ifAbsent: