ewma  fixed. Handle xts in the future

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/expectedreturns@13 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
isSymmetric() instead of identical()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/expectedreturns@14 edb9625f-4e0d-4859-8d74-9fd3b1da38cb