fix: text output of 8-bit integers fixed (resolves #470)

src/core-io/include/mp-units/ostream.h

@@ -34,7 +34,11 @@ namespace detail {
 template<typename CharT, class Traits, auto R, typename Rep>
 void to_stream(std::basic_ostream<CharT, Traits>& os, const quantity<R, Rep>& q)
 {
-  os << q.number();
+  if constexpr (is_same_v<Rep, std::uint8_t> || is_same_v<Rep, std::int8_t>)
+    // promote the value to int
+    os << +q.number();
+  else
+    os << q.number();
   if constexpr (has_unit_symbol(get_unit(R))) {
     os << " ";
     unit_symbol_to<CharT>(std::ostream_iterator<CharT>(os), get_unit(R));