Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(RemoveDelimiterBug);

function RemoveDelimiterBug(sender, eventArgs) {
    /// <summary>Description</summary>
    /// <param name="sender" type="String">Description of sender</param>
    /// <param name="eventArgs" type="String">Description of eventArgs</param>

    var bodyString = new String();

    bodyString = eventArgs.get_request().get_body();
    bodyString = bodyString.trim();

    if (bodyString.endsWith("&")) {
        eventArgs.get_request().set_body(bodyString.substring(0, bodyString.length - 1));
    }
}