Add FileNode::keys() missed during refactoring

modules/core/src/persistence.cpp

@@ -2083,6 +2083,19 @@ FileNode FileNode::operator[](int i) const
     return *it;
 }
 
+std::vector<String> FileNode::keys() const
+{
+    CV_Assert(isMap());
+
+    std::vector<String> res;
+    res.reserve(size());
+    for (FileNodeIterator it = begin(); it != end(); ++it)
+    {
+        res.push_back((*it).name());
+    }
+    return res;
+}
+
 int FileNode::type() const
 {
     const uchar* p = ptr();