Problem: No test for memoization

Solution: Add tests for `to_dict` and `from_dict` memoization

tests/common/test_memoize.py

@@ -0,0 +1,67 @@
+# Copyright BigchainDB GmbH and BigchainDB contributors
+# SPDX-License-Identifier: (Apache-2.0 AND CC-BY-4.0)
+# Code is Apache-2.0 and docs are CC-BY-4.0
+
+import pytest
+from copy import deepcopy
+
+from bigchaindb.models import Transaction
+from bigchaindb.common.crypto import generate_key_pair
+from bigchaindb.common.memoize import to_dict, from_dict
+
+
+pytestmark = [pytest.mark.tendermint, pytest.mark.bdb]
+
+
+def test_memoize_to_dict(b):
+    alice = generate_key_pair()
+    asset = {
+        'data': {'id': 'test_id'},
+    }
+
+    assert to_dict.cache_info().hits == 0
+    assert to_dict.cache_info().misses == 0
+
+    tx = Transaction.create([alice.public_key],
+                            [([alice.public_key], 1)],
+                            asset=asset,)\
+                    .sign([alice.private_key])
+
+
+    tx.to_dict()
+
+    assert to_dict.cache_info().hits == 0
+    assert to_dict.cache_info().misses == 1
+
+    tx.to_dict()
+    tx.to_dict()
+
+    assert to_dict.cache_info().hits == 2
+    assert to_dict.cache_info().misses == 1
+
+
+def test_memoize_from_dict(b):
+    alice = generate_key_pair()
+    asset = {
+        'data': {'id': 'test_id'},
+    }
+
+    assert from_dict.cache_info().hits == 0
+    assert from_dict.cache_info().misses == 0
+
+    tx = Transaction.create([alice.public_key],
+                            [([alice.public_key], 1)],
+                            asset=asset,)\
+                    .sign([alice.private_key])
+    tx_dict = deepcopy(tx.to_dict())
+
+    Transaction.from_dict(tx_dict)
+
+    assert from_dict.cache_info().hits == 0
+    assert from_dict.cache_info().misses == 1
+
+    Transaction.from_dict(tx_dict)
+    Transaction.from_dict(tx_dict)
+
+    assert from_dict.cache_info().hits == 2
+    assert from_dict.cache_info().misses == 1