Matching date axes don't autoscale correctly

[Toumetis]

doAutoRangeAndConstraints attempts to take the min/max of the string representation, breaking autorange / reset axis.

https://codepen.io/Toumetis/pen/eoGogx

My fix is here, please review. Toumetis@dc9b25e

[etpinard]

Thanks very much for the report!

At first glance, your commit looks right. Would you mind making a PR? For bonus point, you could try adding a data/layout mock to lock down the behavior (see https://github.com/plotly/plotly.js/tree/master/test/image#c-generate-or-update-existing-baseline-image for more info)