skip output check if output not defined on formatter

Author: niceking

lib/rspec/core/formatters.rb

@@ -194,7 +194,9 @@ def register(formatter, notifications)
 
     def duplicate_formatter_exists?(new_formatter)
       @formatters.any? do |formatter|
-        formatter.class == new_formatter.class && formatter.output == new_formatter.output
+        formatter.class == new_formatter.class &&
+          formatter.respond_to?(:output) == new_formatter.respond_to?(:output) &&
+          (formatter.respond_to?(:output) ? formatter.output == new_formatter.output : true)
       end
     end