Fixes when given an array as `rspec_opt`
Created by: marcandre
Rspec raketask's rspec_opt
accepts a string or an array of strings.
In Ruby 2.7 a deprecation warning is generated for arrays. This led me to find and error that was undetected because there is no spec for the array case. Moreover I realized that the existing spec for string was erroneously written. This PR fixes both issues