Fix Spliterator implementations for sorted collections

Spliterators returned by CollectSpliterators.indexed and ImmutableSortedSet.spliterator
violated the Spliterator API by not returning null in getComparator when the source
items are naturally sorted. The effect was that sort operations on Streams backed by
these Spliterators were not optimized away, resulting in additional unnecessary sorting.

Fixes google#6187.

guava/src/com/google/common/collect/CollectSpliterators.java

@@ -96,7 +96,7 @@ public int characteristics() {
       @CheckForNull
       public Comparator<? super T> getComparator() {
         if (hasCharacteristics(Spliterator.SORTED)) {
-          return comparator;
+          return Ordering.natural().equals(comparator) ? null : comparator;
         } else {
           throw new IllegalStateException();
         }

guava/src/com/google/common/collect/ImmutableSortedSet.java

@@ -805,8 +805,9 @@ public boolean tryAdvance(Consumer<? super E> action) {
       }
 
       @Override
+      @CheckForNull
       public Comparator<? super E> getComparator() {
-        return comparator;
+        return Ordering.natural().equals(comparator) ? null : comparator;
       }
     };
   }