Issue #486: append a missing ".0" to negative double values too.

tests/test_double_serializer.c

@@ -74,4 +74,8 @@ int main()
 		printf("ERROR: json_c_set_serialization_double_format() failed");
 
 	json_object_put(obj);
+
+	obj = json_object_new_double(-12.0);
+	printf("obj(-12.0).to_string(default format)=%s\n", json_object_to_json_string(obj));
+
 }

tests/test_double_serializer.expected

@@ -16,3 +16,4 @@ obj(12.0).to_string(default format)=12.0
 obj(12.0).to_string(%.0f)=12
 obj(12.0).to_string(%.0g)=1e+01
 obj(12.0).to_string(%.1g)=12.0
+obj(-12.0).to_string(default format)=-12.0