wrong trade price if stop_price is set too low

[sidgwick]

related issue #174

If you change the test line 484 below

backtesting.py/backtesting/test/_test.py

Lines 482 to 489 in c21d7d9

	def test_stop_limit_order_price_is_stop_price(self):
	def coroutine(self):
	self.buy(stop=112, limit=113, size=1)
	self.sell(stop=107, limit=105, size=1)
	yield
	stats = self._Backtest(coroutine).run()
	self.assertListEqual(stats._trades.filter(like='Price').stack().tolist(), [112, 107])

to

self.buy(stop=88, limit=113, size=1)

backtesting framework will make a new trade with price 88, but testing google price never reach that price.