fixed creation of opponent space descriptors (#1805)

modules/features2d/src/descriptors.cpp

@@ -99,7 +99,8 @@ Ptr<DescriptorExtractor> DescriptorExtractor::create(const string& descriptorExt
     if( descriptorExtractorType.find("Opponent") == 0 )
     {
         size_t pos = string("Opponent").size();
-        return DescriptorExtractor::create(descriptorExtractorType.substr(pos));
+        string type = descriptorExtractorType.substr(pos);
+        return new OpponentColorDescriptorExtractor(DescriptorExtractor::create(type));
     }
     
     return Algorithm::create<DescriptorExtractor>("Feature2D." + descriptorExtractorType);