code: func (hs *HmacSigner) Sign(buf []byte) ([]byte, error) {
  	sig, err := hs.SignBytes(buf)
  	if err != nil {
  		return nil, err
  	}
  
  	enc := make([]byte, b64.EncodedLen(len(sig)))
  	b64.Encode(enc, sig)
  
  	return enc, nil
  }
docs: // Sign creates a signature for buf, returning it as a URL-safe base64 encoded
  // byte slice.