(IAC-1228) Remove '+' from shell_join_spec test data

Prior to this commit, the spec/functions/shell_join_spec.rb tests
were failing on Ruby 2.7 as the Shellwords.shelljoin method in
2.7 does not escape the + character.

Rather than have specific datasets for Ruby versions < 2.7 and
>= 2.7, it seemed easiest to simply remove this character from
the tests.