Fix failing test from #269

This also addresses #262 by defaulting to the file extension's parser
when the parser is unknown and allows RDoc to fall back to using the
:markup: directive as a class comment directive in addition of a special
file handling directive.

Fixes #262

Author: drbrain

lib/rdoc/parser.rb

@@ -268,9 +268,11 @@ def self.use_markup content
 
     markup = Regexp.escape markup
 
-    RDoc::Parser.parsers.find do |_, parser|
+    _, parser = RDoc::Parser.parsers.find do |_, parser|
       /^#{markup}$/i =~ parser.name.sub(/.*:/, '')
-    end.last
+    end
+
+    parser
   end
 
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