title: How encode []rune into []byte using utf8
body: <p>It's really easy to decode a <code>[]byte</code> into a <code>[]rune</code> (simply cast to <code>string</code>, then cast to <code>[]rune</code> works very nicely, I'm assuming it defaults to utf8 and with filler bytes for invalids). My question is - how are you suppose to decode this <code>[]rune</code> back to <code>[]byte</code> in utf8 form?</p>
  <p>Am I missing something or do I have manually call <a href="http://golang.org/pkg/unicode/utf8/#EncodeRune" rel="noreferrer">EncodeRune</a> for every single rune in my <code>[]rune</code>? Surely there is an encoder that I can simply pass a <code>Writer</code> to.</p>