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/ml/src/lr.cpp

@@ -547,6 +547,7 @@ void LogisticRegressionImpl::write(FileStorage& fs) const
     {
         CV_Error(CV_StsBadArg,"file can't open. Check file path");
     }
+    writeFormat(fs);
     string desc = "Logisitic Regression Classifier";
     fs<<"classifier"<<desc.c_str();
     fs<<"alpha"<<this->params.alpha;