Cache: add method to get a copy of the map

cache/cache_test.go

@@ -31,6 +31,8 @@ func TestCache(t *testing.T) {
 
 	err := c.Set("a", "b")
 	checkErr(err)
+	err = c.Set("p", "q")
+	checkErr(err)
 
 	val, err := c.Get("a")
 	checkErr(err)
@@ -38,6 +40,11 @@ func TestCache(t *testing.T) {
 		log.Panicf("value %v", val)
 	}
 
+	cc := c.Copy()
+	if len(cc) != 2 {
+		log.Panicf("expected 2 items")
+	}
+
 	err = c.Delete("a")
 	checkErr(err)