Get a string key buffer.
function string(obj, vonly) {
  var vtype = this.type(0)
    , key = !vonly ? this.getStringBuffer(obj.key) : new Buffer(0)
    , value = this.getStringBuffer(obj.value);
  return Buffer.concat(
    [
      vtype,
      key,
      value
    ],
    vtype.length + key.length + value.length
  )
}