Fix the sample usage in the README

If you follow the example in the README you may not have implemented the
:key_value_separator parameter in your type, and you get the wonderful failure
case:

   Puppet::Error: Invalid parameter key_val_separator(:key_val_separator)

This change looks first, and if the user hasn't specified that attribute in
their type in their type, it falls back to '='

lib/puppet/provider/ini_setting/ruby.rb

@@ -83,7 +83,11 @@ Puppet::Type.type(:ini_setting).provide(:ruby) do
   end
 
   def separator
-    resource[:key_val_separator] || '='
+    if resource.class.validattr?(:key_val_separator)
+      resource[:key_val_separator] || '='
+    else
+      '='
+    end
   end
 
   private