Fix Style/CaseLikeIf Rubocop offence
Fix Style/CaseLikeIf Rubocop offence

When building a `Restforce::Mash`, this switches to understanding
what the input value is using a `case` statement rather than
complex `if`/`else` clauses. In particular, we check the type
of the value using `case value when Array` rather than `if
value.is_a?(Array)`.

ejholmes/restforce,restforce/restforce,ejholmes/restforce,restforce/restforce