Honor default separator in set_config

... to use the consistent separator by default in all functions by
default. Users can still override the separator by passing
the separator argument when calling the function.

Fixes: puppetlabs#544

lib/puppet/util/ini_file.rb

@@ -63,6 +63,7 @@ def set_value(*args)
       when 3
         # Backwards compatible set_value function, See MODULES-5172
         (section_name, setting, value) = args
+        separator = @key_val_separator
       when 4
         (section_name, setting, separator, value) = args
       end