Add immutable sellIn behavior

SellIn does not change. Ever.

Java/src/main/java/com/gildedrose/behavior/sellin/ImmutableSellInBehavior.java

@@ -0,0 +1,15 @@
+package com.gildedrose.behavior.sellin;
+
+import com.gildedrose.Item;
+
+public class ImmutableSellInBehavior implements SellInBehavior {
+
+    public static ImmutableSellInBehavior newInstance() {
+        return new ImmutableSellInBehavior();
+    }
+
+    @Override
+    public void processSellInUpdate(Item item) {
+
+    }
+}

Java/src/test/java/com/gildedrose/behavior/sellin/ImmutableSellInBehaviorTest.java

@@ -0,0 +1,45 @@
+package com.gildedrose.behavior.sellin;
+
+import com.gildedrose.Item;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ImmutableSellInBehaviorTest {
+
+    private SellInBehavior sellInBehavior;
+
+    @BeforeEach
+    public void setUp() throws Exception {
+        sellInBehavior = new ImmutableSellInBehavior();
+    }
+
+    @Test
+    void immutableSellIn() {
+        Item item = getItem(10);
+        sellInBehavior.processSellInUpdate(item);
+
+        assertEquals(10, item.sellIn);
+    }
+
+    @Test
+    void immutableSellInZero() {
+        Item item = getItem(0);
+        sellInBehavior.processSellInUpdate(item);
+
+        assertEquals(0, item.sellIn);
+    }
+
+    @Test
+    void immutableNegativeSellIn() {
+        Item item = getItem(-1);
+        sellInBehavior.processSellInUpdate(item);
+
+        assertEquals(-1, item.sellIn);
+    }
+
+    private Item getItem(int sellIn) {
+        return new Item("SomeItem", sellIn, 0);
+    }
+}