Focus ring now shows up around custom navigation buttons

[nkinser]

This PR fixes an issue where when custom navigation buttons are provided, the focus ring for keyboard-only users did not appear around the custom buttons. This issue is fixed by only supplying a tabindex when the default navigation buttons are used. This means that in order for custom navigation buttons to receive keyboard focus a tabindex must be specified on the object passed in.

Before

After

[coveralls]

Coverage increased (+0.03%) to 84.529% when pulling a84e51a on nkinser:nk--fix-custom-nav-buttons-focus-ring into dd1ab72 on airbnb:master.

[ljharb]

Seems good; worth a regression test?

[nkinser]

@ljharb How would I add regression tests for this?
I do think this could be considered a breaking change if anyone supplies custom navigation buttons, this new change will cause the buttons to be skipped over with keyboard navigation.

[ljharb]

Hmm, that's a fair point.

As for a test, I'd assume we could provide one test that uses default buttons, and asserts the tabIndex is present, and another that uses custom ones, and asserts that it's not?

Even better would be adding those test cases to master first, and then updating them in this PR, which would make the breakage very clear.

[nkinser]

@ljharb Would it be possible to check in DayPickerNavigation if the supplied custom buttons have a tabIndex and then only omit the tabIndex prop if the custom buttons have it? That way it wouldn't be a breaking change

[ljharb]

Not reliably, no - all we can do is render what they give us.

[nkinser]

@ljharb do the test changes look good to you?

[ljharb]

Looks great, thanks!