Fix PowerShell encoding from ASCII-8BIT to UTF-16LE for NON-ASCII string...

see https://github.com/WinRb/vagrant-windows/issues/182.

In `winrm.winrm_service.run_powershell_script`, script has encoded by `ASCII-8BIT`. When script contains NON-ASCII string, that code raises Unicode Error. So we can't do provisioning successfully.

This PR allows us to do provisioning by powershell like `netsh interface ip set address \"ローカル エリア接続\" static 192.168.56.101 255.255.255.0"`.

Please test this PR.