jsx-eslint#969 fix: exception for jsx-no-duplicate-props

jsx-no-duplicate-props is causing error. 
TypeError: name.toLowerCase is not a function. 
When <Element {...props}> is used.

lib/rules/jsx-no-duplicate-props.js

@@ -45,7 +45,7 @@ module.exports = {
 
           let name = decl.name.name;
 
-          if (ignoreCase) {
+          if (ignoreCase && typeof name === 'string') {
             name = name.toLowerCase();
           }