Feature/add test for attendancelist update (#2)

tests/test_attendancelist.py

@@ -1,4 +1,4 @@
-from maorganizer.datawrangling import Person, Attendancelist
+from maorganizer.datawrangling import Attendancelist, Person
 
 
 def test_attendancelist_finds_person_by_substring():
@@ -20,3 +20,15 @@ def test_find_multiple_finds_alls():
         Person(name="Ford Prefix"): {Person(name="Ford Prefix")},
         Person(name="Zaphod"): {Person(name="Zaphod Beeblebrox")},
     }
+
+
+def test_attendance_list_updates_correctly():
+    assert Attendancelist({Person("zaphod beeblebrox"), Person("ford prefix")}).update(
+        Attendancelist(
+            {
+                Person("zaphod beeblebrox"),
+                Person("ford prefix"),
+                Person("lord crawfish"),
+            }
+        )
+    ) == Attendancelist(participants={Person("lord crawfish")})