resolve compiler warnings (#1246)
resolve compiler warnings (#1246)

* resolve compiler warnings

- check `WSAStartup` return.
- `const` is not suitable for `std::move`.

* resolve compiler warnings

- bool startup => bool is_valid_.
- remove `const` not removed.
yhirose/cpp-httplib,yhirose/cpp-httplib,yhirose/cpp-httplib,yhirose/cpp-httplib