message: Enable HTTP/2 on Windows 10
  Winhttp is capable of using HTTP/2 on Windows 10, version 1607 or
  later, but it needs to be explicitly enabled. Do it and also define the
  necessary constants ourselves if they are missing so that compilation
  with an older SDK is full-featured.
diff: // can't include both winhttp.h and wininet.h, so put a declaration here
  //#include <wininet.h>
  EXTERN_C DECLSPEC_IMPORT BOOL STDAPICALLTYPE InternetGetConnectedState(__out LPDWORD lpdwFlags, __reserved DWORD dwReserved);
  +
  + #ifndef WINHTTP_OPTION_ENABLE_HTTP_PROTOCOL
  + #define WINHTTP_OPTION_ENABLE_HTTP_PROTOCOL 133
  + #endif
  + #ifndef WINHTTP_PROTOCOL_FLAG_HTTP2
  + #define WINHTTP_PROTOCOL_FLAG_HTTP2 0x1
  + #endif
  #endif
  namespace
  @@ -297,6 +304,12 @@ private:
  c.set_proxy(uri(L"//" + std::wstring(ieConfig.lpszProxy)));
  }
  }
  +
  + c.set_nativesessionhandle_options([](http::client::native_handle handle){
  + DWORD dwOption = WINHTTP_PROTOCOL_FLAG_HTTP2;
  + WinHttpSetOption(handle, WINHTTP_OPTION_ENABLE_HTTP_PROTOCOL, &dwOption, sizeof(dwOption));
  + });
  +
  return c;
  }
  