message: get rid of warning C4800 under VS2010
  the "std::basic_ios::operator bool" differs between C++98 and C++11. The
  "double not" syntax is portable and covers both cases with equivalent
  meaning
diff: @@ -7697,7 +7697,7 @@ std::string base64::make_base64_header(const char * dt)
  bool base64::read_base64_header(std::vector<char> const & header, std::string & dt)
  {
  std::istringstream iss(header.data());
  - return static_cast<bool>(iss >> dt);
  + return !!(iss >> dt);//the "std::basic_ios::operator bool" differs between C++98 and C++11. The "double not" syntax is portable and covers both cases with equivalent meaning
  }
  /****************************************************************************
  