test: convert the example in issue allegro#300 into a test case

The new commit-id 16df11e can fix the issue

bigcache_test.go

@@ -1132,3 +1132,29 @@ func TestCache_RepeatedSetWithBiggerEntry(t *testing.T) {
 	}
 
 }
+
+// TestBigCache_allocateAdditionalMemoryLeadPanic
+// The new commit 16df11e change the encoding method,it can fix issue #300
+func TestBigCache_allocateAdditionalMemoryLeadPanic(t *testing.T){
+	t.Parallel()
+	cache, _ := NewBigCache(Config{
+		Shards:       1,
+		LifeWindow:   3 * time.Second,
+		MaxEntrySize: 52,
+	})
+	cache.Set("a",  blob(0xff, 235))
+	time.Sleep(2 * time.Second)
+	cache.Set("b",  blob(0xff, 235))
+	// expire the first element
+	time.Sleep(2 * time.Second)
+	// insert the before head
+	cache.Set("c", blob(0xff, 108))
+	// reallocate the memory
+	cache.Set("d",  blob(0xff, 1024))
+	time.Sleep(4 * time.Second)
+	cache.Set("e",	blob(0xff, 3))
+	cache.Set("f", blob(0xff, 3))
+	cache.Set("g", blob(0xff, 3))
+	data,_:= cache.Get("g")
+	assertEqual(t, []byte{0xff,0xff,0xff}, data)
+}