String-symbol parameter defaults are incorrectly shown as identifiers

[AaronC81]

If a string-symbol (:'foo') is used as a default value for a method parameter, then that default is incorrectly converted to a plain identifier (foo). This was mentioned on an issue on my project which uses YARD's registry.

Steps to reproduce

Save the following into a file called main.rb.

module Example
  class Foo
    def bar(x = :'abomination')
      3
    end
  end
end

Then:

1. Run the following command: yardoc main.rb
2. Open the docs for the only method

Actual Output

Expected Output

The signature should have the default as :'abomination' rather than abomination.

Environment details:

• OS: Ubuntu 18.04
• Ruby version (ruby -v): Ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]
• YARD version (yard -v): yard 0.9.19
• Relevant software dependency/versions: N/A

I have read the Contributing Guide.