ASP.NET MVC 3 LoadingElementDuration is not working

jQuery expects a numeric value load duration. However unobstrusive ajax
passes it the attribute value (a string type) when it is set. This
change ensures we parse the value as a decimal if it's set.

Work Item: 1520

(cherry picked from commit 8fdce28acf3c8df82dcb02581e2929c5758bd173)