Fix missing format when writing Algorithm-based objects

Added a writeFormat() method to Algorithm which must be called by the
write() method of derived classes.

modules/core/include/opencv2/core.hpp

@@ -3059,6 +3059,9 @@ public:
     /** Returns the algorithm string identifier.
      This string is used as top level xml/yml node tag when the object is saved to a file or string. */
     CV_WRAP virtual String getDefaultName() const;
+
+protected:
+    void writeFormat(FileStorage& fs) const;
 };
 
 struct Param {

modules/core/src/algorithm.cpp

@@ -57,7 +57,6 @@ void Algorithm::save(const String& filename) const
 {
     FileStorage fs(filename, FileStorage::WRITE);
     fs << getDefaultName() << "{";
-    fs << "format" << (int)3;
     write(fs);
     fs << "}";
 }
@@ -67,6 +66,11 @@ String Algorithm::getDefaultName() const
     return String("my_object");
 }
 
+void Algorithm::writeFormat(FileStorage& fs) const
+{
+    fs << "format" << (int)3;
+}
+
 }
 
 /* End of file. */