message: added char cast
  should fix the GCC unittest
diff: @@ -415,7 +415,7 @@ class serializer
  {
  // we finish reading, but do not accept: string was incomplete
  std::string sn(3,'\0');
  - snprintf(&sn[0], sn.size(), "%.2X", s.back());
  + snprintf(&sn[0], sn.size(), "%.2X", static_cast<uint8_t>(s.back()));
  JSON_THROW(type_error::create(316, "incomplete UTF-8 string; last byte: 0x" + sn));
  }
  }
  