Do not constantize Spree.user_class in UserClassHandle

`Spree.user_class` will constantize the configured user class. If called
during Rails start-up, this will autoload the configured user class,
which is not necessarily desired. We can do without the autoloading by
directly getting the class variable rather than going through
`Spree.user_class`.

core/lib/spree/user_class_handle.rb

@@ -21,8 +21,8 @@ class UserClassHandle
     # @return [String] the name of the user class as a string.
     # @raise [RuntimeError] if Spree.user_class is nil
     def to_s
-      fail "'Spree.user_class' has not been set yet." unless Spree.user_class
-      "::#{Spree.user_class}"
+      fail "'Spree.user_class' has not been set yet." unless Spree.class_variable_get?(:@@user_class)
+      "::#{Spree.class_variable_get(:@@user_class)}"
     end
   end
 end