BUG: Fixes reading and writing of daily bars first_trading_day attr

When writing first_trading_day, it is already in the correct frame of
reference (seconds since epoch) and does not need to be transformed
further. Adjusts the reader to expect this value.

tests/data/test_us_equity_pricing.py

@@ -186,6 +186,12 @@ class BcolzDailyBarTestCase(WithBcolzDailyBarReader, ZiplineTestCase):
             DatetimeIndex(result.attrs['calendar'], tz='UTC'),
         )
 
+    def test_read_first_trading_day(self):
+        self.assertEqual(
+            self.bcolz_daily_bar_reader.first_trading_day,
+            self.trading_days[0],
+        )
+
     def _check_read_results(self, columns, assets, start_date, end_date):
         results = self.bcolz_daily_bar_reader.load_raw_arrays(
             columns,