(Maint) Do not overwrite file output

The windows spec code resets the output streams so that it can handle
color correctly. In doing so it assumed that everything was writing to
stdout, which is not the case when -o is used on the command line.

spec/spec_helper.rb

@@ -67,7 +67,12 @@ module PuppetSpec
   if Puppet::Util::Platform.windows?
     config.output_stream = $stdout
     config.error_stream = $stderr
-    config.formatters.each { |f| f.instance_variable_set(:@output, $stdout) }
+
+    config.formatters.each do |f|
+      if not f.instance_variable_get(:@output).kind_of?(::File)
+        f.instance_variable_set(:@output, $stdout)
+      end
+    end
   end
 
   Puppet::Test::TestHelper.initialize