Fix rdoc_top_level_for for RDoc 6.13.0

ruby/rdoc@f517b02 changed the signature
of `RDoc::Store#initialize` from:

  ```ruby
  def initialize(path = nil, type = nil)
  ```

to:

  ```ruby
  def initialize(options, path: nil, type: nil)
  ```

which is a breaking change.

This commit modifies `rdoc_top_level_for` to use the new signature.
Because the issue only affects our testing code, this commit ignores
compatibility with RDoc < 6.13.0.

Author: jonathanhefner

spec/spec_helper.rb

@@ -28,7 +28,7 @@ def rdoc_top_level_for(ruby_code)
   # foolproof way to initialize it is by simply running it with a dummy file.
   $rdoc_for_specs ||= rdoc_dry_run("--files", __FILE__)
 
-  $rdoc_for_specs.store = RDoc::Store.new
+  $rdoc_for_specs.store = RDoc::Store.new(RDoc::Options.new)
 
   Dir.mktmpdir do |dir|
     path = "#{dir}/ruby_code.rb"