Fixing plus operator which caused OHLCV failue

Fixes issue with passing in a "since" timestamp will cause the request "end" to look like
    [id] => BTC-USD
    [granularity] => 60
    [start] => 2017-12-23T18:50:59.000+00:00
    [end] => 57041747-04-17T02:50:59.000+00:00

Which throws this error
ccxt\ExchangeError: gdax {"message":"Invalid interval"}

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