Fix TreeMultimap Java map conversion (#3132)

The Java conversion of `TreeMultimap` did not preserve the original comparator, resulting in incorrect key ordering

Author: pivovarit

vavr/src/main/java/io/vavr/collection/TreeMultimap.java

@@ -954,6 +954,6 @@ public SortedSet<K> keySet() {
 
     @Override
     public java.util.SortedMap<K, Collection<V>> toJavaMap() {
-        return toJavaMap(new java.util.TreeMap<>());
+        return toJavaMap(new java.util.TreeMap<>(comparator()));
     }
 }

vavr/src/test/java/io/vavr/collection/TreeMultimapTest.java

@@ -4,12 +4,14 @@
 import java.math.BigDecimal;
 import java.util.ArrayList;
 import java.util.Comparator;
+import java.util.LinkedList;
 import java.util.Map;
 import java.util.Spliterator;
 import java.util.function.Function;
 import java.util.function.Supplier;
 import java.util.stream.Collector;
 import java.util.stream.Stream;
+import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.TestTemplate;
 
 public class TreeMultimapTest extends AbstractMultimapTest {
@@ -196,6 +198,21 @@ protected <K extends Comparable<K>, V> Multimap<K, V> mapFill(int n, Tuple2<? ex
                 throw new RuntimeException();
         }
     }
+    
+    // -- toJavaMap
+    
+    @TestTemplate
+    public void shouldReturnJavaMapSortedUsingSameComparator() {
+        final TreeMultimap<String, String> vavrMap = TreeMultimap.withSeq().of(Comparator.comparing(String::length), "aa", "aa", "b", "b");
+
+        final LinkedList<String> orderedJavaMapKeys = new LinkedList<>();
+        final LinkedList<String> orderedVavrMapKeys = new LinkedList<>();
+        vavrMap.forEach((k,v) -> orderedVavrMapKeys.add(k));
+        vavrMap.toJavaMap().forEach((k, v) -> orderedJavaMapKeys.add(k));
+        
+        Assertions.assertThat(orderedJavaMapKeys).containsExactly("b", "aa");
+        Assertions.assertThat(orderedVavrMapKeys).containsExactly("b", "aa");
+    }
 
     // -- static narrow