Add more merge test cases

These were supposed to fail, but unfortunately didn't, so the bug is not
as simple as the test cases try to check. However, I'll still leave them
in since they're worth having. The bug described in #2 is probably
elsewhere.

tests/test_analysis.py

@@ -51,4 +51,22 @@ def test_merge_price_history():
     assert item['price'] == 2
     assert len(item['priceHistory']) == 2
     assert item['priceHistory'][0]['price'] == 2
-    assert item['priceHistory'][1]['price'] == 3
+    assert item['priceHistory'][1]['price'] == 3
+
+    # Repeat run of history on same day
+    old_items = [get_item()]
+    new_items = [old_items[0]]
+    items = analysis.merge_price_history(old_items, new_items)
+    assert len(items) == 1
+    item = items[0]
+    assert len(item['priceHistory']) == 1
+
+    # Prices with history don't get overwritten
+    oldest_items = [get_item()]
+    middle_items = [get_item(price=2)]
+    newest_items = [get_item(price=2)]
+    items = analysis.merge_price_history(oldest_items, middle_items)
+    items = analysis.merge_price_history(items, newest_items)
+    assert len(items) == 1
+    item = items[0]
+    assert len(item['priceHistory']) == 2