Add basic test for Asset.calls property

requirements-test.txt

@@ -4,4 +4,5 @@ scrypt==0.7.1
 Events==0.2.2
 pyyaml
 pytest
+pytest-mock
 coverage

tests/test_asset.py

+from bitshares.asset import Asset
+from bitshares import BitShares
+
+
+def test_calls(mocker):
+    asset = Asset("USD", lazy=True, bitshares_instance=BitShares(offline=True))
+    method = mocker.patch.object(Asset, 'get_call_orders')
+    asset.calls
+    method.assert_called_with(10)