Add support for dense.cwiseProduct(sparse)

This also fixes a regression regarding (dense*sparse).diagonal()

test/sparse_basic.cpp

@@ -188,6 +188,8 @@ template<typename SparseMatrixType> void sparse_basic(const SparseMatrixType& re
     refM4.setRandom();
     // sparse cwise* dense
     VERIFY_IS_APPROX(m3.cwiseProduct(refM4), refM3.cwiseProduct(refM4));
+    // dense cwise* sparse
+    VERIFY_IS_APPROX(refM4.cwiseProduct(m3), refM4.cwiseProduct(refM3));
 //     VERIFY_IS_APPROX(m3.cwise()/refM4, refM3.cwise()/refM4);
 
     // test aliasing