Removed bug in ouput of FieldMatrix<K,1,1>.

[[Imported from SVN: r2094]]

istl/io.hh

@@ -158,7 +158,7 @@ namespace Dune {
     {
       s << " ";                   // space in front of each entry
       s.width(width);             // set width for each entry anew
-      s << A();                   // yeah, the number !
+      s << static_cast<const K&>(A);                   // yeah, the number !
     }
   }