Add explicit ctor for diagonal to sparse conversion
diff --git a/test/sparse_basic.cpp b/test/sparse_basic.cpp
index 2ebf4d4..492b3a4 100644
--- a/test/sparse_basic.cpp
+++ b/test/sparse_basic.cpp
@@ -372,6 +372,8 @@
     SparseMatrixType m2(rows, rows);
     m2 = d.asDiagonal();
     VERIFY_IS_APPROX(m2, refMat2);
+    SparseMatrixType m3(d.asDiagonal());
+    VERIFY_IS_APPROX(m3, refMat2);
     refMat2 += d.asDiagonal();
     m2 += d.asDiagonal();
     VERIFY_IS_APPROX(m2, refMat2);