IEX Market Data functions not filtering when symbol passed

[addisonlynch]

get_market_tops and get_market_last should restrict their results to a single symbol or list of symbols if they are passed, else return data for all listed symbols. Thus we would expect:

from iexfinance import get_market_tops
len(get_market_tops("AAPL"))

to be 1. But as of now both this function and get_market_last return the entire list (~8600 tickers) each time.