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/features2d/src/blobdetector.cpp

@@ -184,6 +184,7 @@ void SimpleBlobDetectorImpl::read( const cv::FileNode& fn )
 
 void SimpleBlobDetectorImpl::write( cv::FileStorage& fs ) const
 {
+    writeFormat(fs);
     params.write(fs);
 }