/balance should use cached tickers when possible

tests/exchange/test_exchange.py

@@ -1319,6 +1319,14 @@ def test_get_tickers(default_conf, mocker, exchange_name):
     assert tickers['ETH/BTC']['ask'] == 1
     assert tickers['BCH/BTC']['bid'] == 0.6
     assert tickers['BCH/BTC']['ask'] == 0.5
+    assert api_mock.fetch_tickers.call_count == 1
+
+    api_mock.fetch_tickers.reset_mock()
+
+    # Cached ticker should not call api again
+    tickers2 = exchange.get_tickers(cached=True)
+    assert tickers2 == tickers
+    assert api_mock.fetch_tickers.call_count == 0
 
     ccxt_exceptionhandlers(mocker, default_conf, api_mock, exchange_name,
                            "get_tickers", "fetch_tickers")

tests/rpc/test_rpc.py

@@ -569,6 +569,8 @@ def test_rpc_balance_handle(default_conf, mocker, tickers):
     result = rpc._rpc_balance(default_conf['stake_currency'], default_conf['fiat_display_currency'])
     assert prec_satoshi(result['total'], 12.309096315)
     assert prec_satoshi(result['value'], 184636.44472997)
+    assert tickers.call_count == 1
+    assert tickers.call_args.kwargs['cached'] is True
     assert 'USD' == result['symbol']
     assert result['currencies'] == [
         {'currency': 'BTC',