DSL Spec: add the frozen string literal comment to the test input
DSL Spec: add the frozen string literal comment to the test input

Before the patch is applied, this test fails like so:

Failures:

  1) Bundle::Dsl processes input
     Failure/Error: instance_eval(@input)

     RuntimeError:
       Invalid Brewfile: can't modify frozen String
     # ./lib/bundle/dsl.rb:30:in `instance_eval'
     # ./lib/bundle/dsl.rb:116:in `sanitize_brew_name'
     # ./lib/bundle/dsl.rb:85:in `brew'
     # (eval):5:in `process'
     # ./lib/bundle/dsl.rb:30:in `instance_eval'
     # ./lib/bundle/dsl.rb:30:in `process'
     # ./lib/bundle/dsl.rb:21:in `initialize'
     # ./spec/dsl_spec.rb:8:in `new'
     # ./spec/dsl_spec.rb:8:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:26:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:26:in `block (2 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # RuntimeError:
     #   can't modify frozen String
     #   ./lib/bundle/dsl.rb:30:in `instance_eval'

Homebrew/homebrew-bundle