title: case insensitive explicit ordering guava
body: <p>Below code returns output 'ladies' correctly because case sensitive.</p>
  
  <pre><code>import com.google.common.collect.SortedSetMultimap;
  import com.google.common.collect.TreeMultimap;
  import com.google.common.collect.Ordering;
  
  private static final Ordering&lt;String&gt; PRODUCT_ORDER = Ordering
              .explicit("men","ladies")
              .reverse();
  
  
  private void testGuavaOrdering() {
          SortedSetMultimap&lt;Integer, String&gt; onlineAreaPriorityCountMap = TreeMultimap
                  .create(Collections.reverseOrder(), PRODUCT_ORDER);
          onlineAreaPriorityCountMap.put(1, "men");
          onlineAreaPriorityCountMap.put(1, "men");
          onlineAreaPriorityCountMap.put(1, "ladies");
          onlineAreaPriorityCountMap.put(1, "ladies");
          System.out.println(onlineAreaPriorityCountMap.values().iterator().next());
      }
  </code></pre>
  
  <p>But, below code not working and throws exception if i change from men to Men. </p>
  
  <pre><code>private static final Ordering&lt;String&gt; PRODUCT_ORDER = Ordering
              .explicit("Men","Ladies")
              .reverse()
  
  
  private void testGuavaOrdering() {
          SortedSetMultimap&lt;Integer, String&gt; onlineAreaPriorityCountMap = TreeMultimap
                  .create(Collections.reverseOrder(), PRODUCT_ORDER);
          onlineAreaPriorityCountMap.put(1, "men");
          onlineAreaPriorityCountMap.put(1, "men");
          onlineAreaPriorityCountMap.put(1, "ladies");
          onlineAreaPriorityCountMap.put(1, "ladies");
          System.out.println(onlineAreaPriorityCountMap.values().iterator().next());
      }
  </code></pre>
  
  <p>Exception:</p>
  
  <pre><code>Exception in thread "main" com.google.common.collect.Ordering$IncomparableValueException: Cannot compare value: men
      at com.google.common.collect.ExplicitOrdering.rank(ExplicitOrdering.java:46)
      at com.google.common.collect.ExplicitOrdering.compare(ExplicitOrdering.java:40)
      at com.google.common.collect.ReverseOrdering.compare(ReverseOrdering.java:38)
      at java.util.TreeMap.compare(TreeMap.java:1295)
      at java.util.TreeMap.put(TreeMap.java:538)
      at java.util.TreeSet.add(TreeSet.java:255)
      at com.google.common.collect.AbstractMapBasedMultimap.put(AbstractMapBasedMultimap.java:195)
      at com.google.common.collect.AbstractSetMultimap.put(AbstractSetMultimap.java:121)
      at com.google.common.collect.TreeMultimap.put(TreeMultimap.java:78)
      at TestMap.testGuavaOrdering(TestMap.java:38)
      at TestMap.main(TestMap.java:32)
  </code></pre>
  
  <p>How to do case insensitive compare in guava? </p>