Fix modulo/remainder of negative zero (#441)

Author: tompng

ext/bigdecimal/bigdecimal.c

@@ -1735,7 +1735,8 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, NULLABLE_BDVALUE *div, NULLABLE_BDVALUE
     }
     if (VpIsZero(a.real)) {
         VALUE zero = BigDecimal_positive_zero();
-        *div = *mod = (NULLABLE_BDVALUE) { zero, DATA_PTR(zero) };
+        *div = (NULLABLE_BDVALUE) { zero, DATA_PTR(zero) };
+        *mod = bdvalue_nullable(a);
         goto Done;
     }
     if (VpIsInf(b.real)) {

test/bigdecimal/test_bigdecimal.rb

@@ -2740,6 +2740,16 @@ def test_divmod_modulo_remainder_infinity
     end
   end
 
+  def test_signed_zero_modulo_remainder
+    [-0.0, +0.0].each do |a|
+      [-1.0, +1.0].each do |b|
+        assert_equal(BigDecimal(a.remainder(b)).to_s, BigDecimal(a).remainder(BigDecimal(b)).to_s)
+        assert_equal(BigDecimal(a.modulo(b)).to_s, BigDecimal(a).modulo(BigDecimal(b)).to_s)
+        assert_equal(a.divmod(b).map {|x| BigDecimal(x)}.inspect, BigDecimal(a).divmod(BigDecimal(b)).inspect)
+      end
+    end
+  end
+
   def test_bsearch_for_bigdecimal
     assert_raise(TypeError) {
       (BigDecimal('0.5')..BigDecimal('2.25')).bsearch