Datetime columns are not properly formatted to be recognizes as datetime

[n8henrie]

Currently, the datetimes are formatted in a way that is not recognized by datasette-vega for plotting with a Date/time type for the axis.

For example, if you have datasette running locally with datasette-vega installed and have a database that includes resting heart rate:

http://localhost:8001/healthkit/rRestingHeartRate#g.mark=line&g.x_column=startDate&g.x_type=temporal&g.y_column=value&g.y_type=quantitative

The plot is blank unless you choose Label as the type for the date data.

The startDate (and creationDate and endDate) columns appear like: 2019-11-14 18:22:18 -0700

If instead the format for this column is changed slightly: 2019-11-14T18:22:18-07:00 they are recognized as proper dates and the charting works as expected.

I have a PR that addresses this issue, will submit shortly.