Changes pet relation in owner

In order to avoid repeated pets in the relation.

domain/src/main/java/es/uvigo/esei/xcs/domain/entities/Owner.java

@@ -5,8 +5,9 @@ import static java.util.Collections.unmodifiableCollection;
 import static java.util.Objects.requireNonNull;
 
 import java.io.Serializable;
-import java.util.ArrayList;
 import java.util.Collection;
+import java.util.HashSet;
+import java.util.Set;
 
 import javax.persistence.CascadeType;
 import javax.persistence.DiscriminatorValue;
@@ -28,12 +29,11 @@ public class Owner extends User implements Serializable {
 	
 	@OneToMany(
 		mappedBy = "owner",
-		targetEntity = Pet.class,
 		cascade = CascadeType.ALL,
 		orphanRemoval = true,
 		fetch = FetchType.EAGER
 	)
-	private Collection<Pet> pets;
+	private Set<Pet> pets;
 	
 	// Required for JPA
 	Owner() {}
@@ -53,7 +53,7 @@ public class Owner extends User implements Serializable {
 	 */
 	public Owner(String login, String password) {
 		super(login, password);
-		this.pets = new ArrayList<>();
+		this.pets = new HashSet<>();
 	}
 	
 	/**
@@ -72,8 +72,7 @@ public class Owner extends User implements Serializable {
 	 * not valid according to its description.
 	 */
 	public Owner(String login, String password, Pet ... pets) {
-		super(login, password);
-		this.pets = new ArrayList<>();
+		this(login, password);
 		
 		stream(pets).forEach(this::addPet);
 	}