fixed bug with simplex initialization in Downhill solver (thanks to Toby Rule for the patch): http://code.opencv.org/issues/3990.

re-enabled Downhill solver test, let's see how it goes

modules/core/src/downhill_simplex.cpp

@@ -354,7 +354,7 @@ namespace cv
         int ndim=_step.cols;
         Mat_<double> simplex=Mat_<double>(ndim+1,ndim,0.0);
 
-        simplex.row(0).copyTo(proxy_x);
+        proxy_x.copyTo(simplex.row(0));
         createInitialSimplex(simplex,_step);
         double res = innerDownhillSimplex(
                 simplex,_termcrit.epsilon, _termcrit.epsilon, count,_Function,_termcrit.maxCount);

modules/core/test/test_downhill_simplex.cpp

@@ -78,7 +78,7 @@ class RosenbrockF:public cv::MinProblemSolver::Function{
     }
 };
 
-TEST(DISABLED_Core_DownhillSolver, regression_basic){
+TEST(Core_DownhillSolver, regression_basic){
     cv::Ptr<cv::DownhillSolver> solver=cv::DownhillSolver::create();
 #if 1
     {