FlashImpl should set a correct path for the Cookies

Currently we do not set a path on the cookies created by FlashImpl which causes problems if the VDL-files are located in different folders, because each cookie is only assigned for its folder and not for the whole context.

Setting the path to externalContext.getRequestContextPath() solves this problem, because it sets each cookie for the whole webapp.

See the thread on the mailing list: http://www.mail-archive.com/dev@myfaces.apache.org/msg48859.html