Remove usage of class Objects

It's possible to use this class on Android only starting with 19 API level.

Author: JinneeJ

commonmark/src/main/java/org/commonmark/internal/ListBlockParser.java

@@ -3,7 +3,6 @@
 import org.commonmark.node.*;
 import org.commonmark.parser.block.*;
 
-import java.util.Objects;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -84,13 +83,17 @@ private static ListData parseListMarker(CharSequence ln, int offset) {
      */
     private static boolean listsMatch(ListBlock a, ListBlock b) {
         if (a instanceof BulletList && b instanceof BulletList) {
-            return Objects.equals(((BulletList) a).getBulletMarker(), ((BulletList) b).getBulletMarker());
+            return equals(((BulletList) a).getBulletMarker(), ((BulletList) b).getBulletMarker());
         } else if (a instanceof OrderedList && b instanceof OrderedList) {
-            return Objects.equals(((OrderedList) a).getDelimiter(), ((OrderedList) b).getDelimiter());
+            return equals(((OrderedList) a).getDelimiter(), ((OrderedList) b).getDelimiter());
         }
         return false;
     }
 
+    private static boolean equals(Object a, Object b) {
+        return (a == null) ? (b == null) : a.equals(b);
+    }
+
     public static class Factory extends AbstractBlockParserFactory {
 
         @Override