parallel_spinach stops working starting in 3.9.0

[tatethurston]

parallel_spinach stops working in 3.9.0 with the following error:

`+': no implicit conversion of String into Array (TypeError)

Ruby: 2.7.8
Rails: 5.2.8.1

parallel_spinach features --verbose-process-command --group-by=filesize

Stack trace:

bundler: failed to load command: parallel_spinach (/Users/tatthurs/.asdf/installs/ruby/2.7.8/bin/parallel_spinach)
Traceback (most recent call last):
        9: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel-1.24.0/lib/parallel.rb:219:in `block (4 levels) in in_threads'
        8: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel-1.24.0/lib/parallel.rb:417:in `block in work_in_threads'
        7: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel-1.24.0/lib/parallel.rb:637:in `with_instrumentation'
        6: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel-1.24.0/lib/parallel.rb:418:in `block (2 levels) in work_in_threads'
        5: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel-1.24.0/lib/parallel.rb:627:in `call_with_index'
        4: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel_tests-4.4.0/lib/parallel_tests/cli.rb:61:in `block (4 levels) in execute_in_parallel'
        3: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel_tests-4.4.0/lib/parallel_tests/cli.rb:85:in `block (2 levels) in run_tests_in_parallel'
        2: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel_tests-4.4.0/lib/parallel_tests/cli.rb:114:in `run_tests'
        1: from /Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel_tests-4.4.0/lib/parallel_tests/gherkin/runner.rb:22:in `run_tests'
/Users/tatthurs/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/parallel_tests-4.4.0/lib/parallel_tests/gherkin/runner.rb:22:in `+': no implicit conversion of String into Array (TypeError)

[grosser]

#938

[grosser]

4.5.1