Fix det_exact for object dtype (#514)

* Fix NMatrix#det_exact for ruby objects

* Add tests for NMatrix#det_exact dtype=:object

ext/nmatrix/ruby_nmatrix.c

@@ -3006,12 +3006,12 @@ static VALUE nm_det_exact(VALUE self) {
   nm::dtype_t dtype = NM_DTYPE(self);
   nm_math_det_exact(NM_SHAPE0(self), NM_STORAGE_DENSE(self)->elements, NM_SHAPE0(self), NM_DTYPE(self), result);
 
+  VALUE to_return;
   if (dtype == nm::RUBYOBJ) {
-    nm_register_values(reinterpret_cast<VALUE*>(result), 1);
-  }
-  VALUE to_return = nm::rubyobj_from_cval(result, NM_DTYPE(self)).rval;
-  if (dtype == nm::RUBYOBJ) {
-    nm_unregister_values(reinterpret_cast<VALUE*>(result), 1);
+    to_return = *reinterpret_cast<VALUE*>(result);
+
+  } else {
+    to_return = nm::rubyobj_from_cval(result, NM_DTYPE(self)).rval;
   }
   NM_CONSERVATIVE(nm_unregister_value(&self));
 

spec/math_spec.rb

@@ -946,7 +946,6 @@
 
   context "determinants" do
     ALL_DTYPES.each do |dtype|
-      next if dtype == :object
       context dtype do
         before do
           @a = NMatrix.new([2,2], [1,2,
@@ -968,13 +967,19 @@
                 end
         end
         it "computes the determinant of 2x2 matrix" do
-          expect(@a.det).to be_within(@err).of(-2)
+          if dtype != :object
+            expect(@a.det).to be_within(@err).of(-2)
+          end
         end
         it "computes the determinant of 3x3 matrix" do
-          expect(@b.det).to be_within(@err).of(-8)
+          if dtype != :object
+            expect(@b.det).to be_within(@err).of(-8)
+          end
         end
         it "computes the determinant of 4x4 matrix" do
-          expect(@c.det).to be_within(@err).of(-18)
+          if dtype != :object
+            expect(@c.det).to be_within(@err).of(-18)
+          end
         end
         it "computes the exact determinant of 2x2 matrix" do
           if dtype == :byte