Add more testing for special values in DoubleDoubleSummation

TEST(Summation, ConvertSpecialToDecimal) {

    constexpr double infinity = std::numeric_limits<double>::infinity();

    DoubleDoubleSummation sum;

    sum.addDouble(infinity);    

    ASSERT_EQUALS(infinity, sum.getDouble()); // ok

    ASSERT_TRUE(sum.getDecimal().isInfinite()); // <-- fail here, it's nan

    ASSERT_FALSE(sum.getDecimal().isNaN()); // <!-- fail here, it is actually nan

    sum.addDouble(1); // now once we add something to sum, everything is ok

    ASSERT_EQUALS(infinity, sum.getDouble()); // ok, still infinity

    ASSERT_TRUE(sum.getDecimal().isInfinite()); // ok, now it's infinity

    ASSERT_FALSE(sum.getDecimal().isNaN()); // ok, not nan

}