vega-lite: Off-by-one error in year-month timeUnit

The following specification has dates starting and ending in the month of June, but the chart renders with data starting and ending in the month of May. Reported in https://github.com/altair-viz/altair/issues/1027

{
  "data": {
    "values": [
      {"date": "2017-06-01", "pct_change_rounded": 0},
      {"date": "2017-07-01", "pct_change_rounded": 0.1},
      {"date": "2017-08-01", "pct_change_rounded": 0.4},
      {"date": "2017-09-01", "pct_change_rounded": 0.5},
      {"date": "2017-10-01", "pct_change_rounded": 0.1},
      {"date": "2017-11-01", "pct_change_rounded": 0.3},
      {"date": "2017-12-01", "pct_change_rounded": 0.2},
      {"date": "2018-01-01", "pct_change_rounded": 0.5},
      {"date": "2018-02-01", "pct_change_rounded": 0.2},
      {"date": "2018-03-01", "pct_change_rounded": -0.1},
      {"date": "2018-04-01", "pct_change_rounded": 0.2},
      {"date": "2018-05-01", "pct_change_rounded": 0.2},
      {"date": "2018-06-01", "pct_change_rounded": 0.1}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {
      "type": "ordinal",
      "axis": {"format": "%b %y"},
      "field": "date",
      "timeUnit": "yearmonth"
    },
    "y": {"type": "quantitative", "field": "pct_change_rounded"}
  }
}