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/akaze.cpp

@@ -217,6 +217,7 @@ namespace cv
 
         void write(FileStorage& fs) const
         {
+            writeFormat(fs);
             fs << "descriptor" << descriptor;
             fs << "descriptor_channels" << descriptor_channels;
             fs << "descriptor_size" << descriptor_size;

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);
 }
 

modules/features2d/src/kaze.cpp

@@ -159,6 +159,7 @@ namespace cv
 
         void write(FileStorage& fs) const
         {
+            writeFormat(fs);
             fs << "extended" << (int)extended;
             fs << "upright" << (int)upright;
             fs << "threshold" << threshold;

modules/features2d/src/matchers.cpp

@@ -1179,6 +1179,7 @@ void FlannBasedMatcher::read( const FileNode& fn)
 
 void FlannBasedMatcher::write( FileStorage& fs) const
 {
+     writeFormat(fs);
      fs << "indexParams" << "[";
 
      if (indexParams)