Support multibyte char for Windows.

I wrote a patch because it fails this test case.

  it "can copy & paste with multibyte char" do
    Encoding.default_external = "utf-8"
    Clipboard.copy("日本語")
    Clipboard.paste.should == "日本語"
  end

Failure/Error: Clipboard.copy("日本語")
Encoding::InvalidByteSequenceError:
  incomplete "\x00" on UTF-16LE