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.
This PR changes the behavior of the else block. Previously it would pad all the characters to work with UTF16, but now it no longer does that. Since the WinRM gem only [supports Ruby 1.9.0 or higher](https://github.com/WinRb/WinRM/blob/master/winrm.gemspec#L26), we can safely remove the check for the `encode` method.

```ruby
script = script_file.kind_of?(IO) ? script_file.read : script_file
script = script.encode('UTF-16LE', 'UTF-8')
script = Base64.strict_encode64(script)
```

I assume the usage of UTF-16LE over UTF-16 is because Windows defaults to little endian?