Support start time in get_prices

[dblock]

Passing ts seems to have no effect, but according to http://www.networkerror.org/component/content/article/1-technical-wootness/44-googles-undocumented-finance-api.html should.

        when :start, :ts then
          query[:ts] = v.to_time.to_i

  context 'with different start', vcr: { cassette_name: 'get_prices_goog_start' } do
    subject do
      GoogleFinance::Prices.get('GOOG', start: Date.parse('2016-1-1 12:00'), fields: %i[date close])
    end
    it 'retrieves prices from 2012' do
      p subject.first
      p subject.last
    end
  end