Unable to modify subRangeSet returned by TreeRangeSet.subRangeSet(Range).
IMO, it should be availiable to do so as specified in the document.

```java
import com.google.common.collect.Range;
import com.google.common.collect.TreeRangeSet;
import org.junit.jupiter.api.Test;

@SuppressWarnings("UnstableApiUsage")
public class SubRangeSetTest {

    @Test
    public void testAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.subRangeSet(range).add(range);
    }

    @Test
    public void testReplaceAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.add(range);
        set.subRangeSet(range).add(range);
    }
}
```

```
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)
	at com.google.common.collect.TreeRangeSet.replaceRangeWithSameLowerBound(TreeRangeSet.java:266)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:216)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testAdd(SubRangeSetTest.java:12)
```
```
java.lang.UnsupportedOperationException
	at com.google.common.collect.UnmodifiableIterator.remove(UnmodifiableIterator.java:46)
	at com.google.common.collect.Iterators.clear(Iterators.java:985)
	at com.google.common.collect.Maps$IteratorBasedAbstractMap.clear(Maps.java:3562)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:214)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testReplaceAdd(SubRangeSetTest.java:20)
```
rangeSet.add(range);
      testEnclosing(rangeSet);
      testEnclosing(rangeSet.complement());
    }
  }
  public void testAllTwoRangesEnclosing() {
    for (Range<Integer> range1 : QUERY_RANGES) {
      for (Range<Integer> range2 : QUERY_RANGES) {
        TreeRangeSet<Integer> rangeSet = TreeRangeSet.create();
        rangeSet.add(range1);
        rangeSet.add(range2);
        testEnclosing(rangeSet);
        testEnclosing(rangeSet.complement());
      }
    }
  }
  public void testCreateCopy() {
    for (Range<Integer> range1 : QUERY_RANGES) {
      for (Range<Integer> range2 : QUERY_RANGES) {
        TreeRangeSet<Integer> rangeSet = TreeRangeSet.create();
        rangeSet.add(range1);
        rangeSet.add(range2);
        assertEquals(rangeSet, TreeRangeSet.create(rangeSet));
      }
    }
  }
  private RangeSet<Integer> expectedSubRangeSet(
      RangeSet<Integer> rangeSet, Range<Integer> subRange) {
    RangeSet<Integer> expected = TreeRangeSet.create();
    for (Range<Integer> range : rangeSet.asRanges()) {