#70 add hashCode for deadline

src/main/java/ru/ulstu/deadline/model/Deadline.java

@@ -9,6 +9,7 @@ import javax.persistence.Entity;
 import javax.persistence.Temporal;
 import javax.persistence.TemporalType;
 import java.util.Date;
+import java.util.Objects;
 
 @Entity
 public class Deadline extends BaseEntity {
@@ -68,4 +69,9 @@ public class Deadline extends BaseEntity {
                 description.equals(deadline.description) &&
                 date.equals(deadline.date);
     }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(super.hashCode(), description, date);
+    }
 }