Fix Puppet::Util::Autoload.load_file to ignore stubbing

I mistakenly added `singleton_class` when checking if the class responds to
`load_file` (to support older versions of Puppet). This is obviously not
the correct method of determining if a class method has been defined and
as such did not work as intended.

This lead to the platform stubbing still being in place when `Kernel.load`
was being called, causing the problems reported by @cdenneen (where
a provider was evaluating `Puppet.features.microsoft_windows?` at load
time).

lib/rspec-puppet/monkey_patches.rb

@@ -213,7 +213,7 @@ def pretending?
     end
 
     class Autoload
-      if singleton_class.respond_to?(:load_file)
+      if respond_to?(:load_file)
         singleton_class.send(:alias_method, :old_load_file, :load_file)
 
         def self.load_file(*args)