PeekIterator -- follow naming convention of java.util.Queue.
  peek() returns element or null
  element() returns element or throws NoSuchElementException.
so old peekOrNull() -> peek() and old peek() -> element().


git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1480582 13f79535-47bb-0310-9956-ffa450edef68