ListItem legend may be a node as well (react-toolbox#1496)

Currently it works, but produces a warning by react typechecking

Author: olegstepura

components/list/ListItemContent.js

@@ -15,7 +15,10 @@ const factory = (ListItemText) => {
         PropTypes.node,
       ]),
       children: PropTypes.node,
-      legend: PropTypes.string,
+      legend: PropTypes.oneOfType([
+        PropTypes.string,
+        PropTypes.node,
+      ]),
       theme: PropTypes.shape({
         auto: PropTypes.string,
         itemContentRoot: PropTypes.string,