Minor fix to avoid a compile time error when using the kernel_matrix() function.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404110

dlib/svm/kernel_matrix.h

@@ -54,7 +54,8 @@ namespace dlib
         // --------------------------------------------
 
         template <typename kernel_type, typename T>
-        inline unsigned long size ( const T& m)
+        inline typename disable_if<is_same_type<T,typename kernel_type::sample_type>,unsigned long>::type 
+        size ( const T& m)
         {
             return m.size();
         }

dlib/test/kernel_matrix.cpp

@@ -141,6 +141,12 @@ namespace
             samp3 = trans(kernel_matrix(kern, samp3, vect2));
             DLIB_TEST(equal(samp3, trans(kernel_matrix(kern, samp, vect2))));
 
+
+            samp2 += kernel_matrix(kern, vect1, samp);
+            DLIB_TEST(equal(samp2, 2*kernel_matrix(kern, vect1, samp)));
+
+            samp3 += trans(kernel_matrix(kern, samp, vect2));
+            DLIB_TEST(equal(samp3, 2*trans(kernel_matrix(kern, samp, vect2))));
         }
     };