fix(CalendarDay): apply styles for start and end modifiers

[tagoro9]

Previous versions of the library would add the CalendarDay--selected-start and CalendarDay--selected-end class names whenselected-start and selected-end modifiers were passed.

I think after the switch to react-with-styles these classes got lost and making it harder to modify the styles of the beginning and end of a range.

I'm not very familiar with react-with-styles, so I don't know if this is the proper way to get those class names set in the component.

I would also like to add unit tests for this, but I didn't see tests in CalendarDay regarding other modifiers and I'm not quite sure how to test this. I see that the wrapper gets the class names in the test, but they have what it seems a random modifier at the end. Also, the react-with-styles docs mention that css(...) returns an object with an opaque structure, so definitely, checking the value of the props added by this object doesn't seem a good testing option.

Fixes #786.

[coveralls]

Coverage remained the same at 86.76% when pulling a4b58d9 on relayrides:calendar_day_start_end_modifiers into ef98109 on airbnb:master.

[majapw]

Hi @tagoro9, you should be able to test if the classes you expect get applied by running npm run storybook:css and inspecting the elements. The default storybook script uses aphrodite which is not quite... the class structure you are expecting.

[tagoro9]

Thanks for the reply @majapw. I tested that the classes got added to the implementation in the story book. When I said I didn't know how to test it, I meant how to write a unit test that ensures that those classes get added. I should have explained that better in the description.

[majapw]

Sweet. Yeah, in general, we don't want to write tests for the styling (because it would be testing the interface's internals) and the end-goal is to integrate with a tool like https://github.com/Galooshi/happo instead to check for visual regressions.

THIS IS ON OUR ... eventual roadmap. :) but i've just been swamped.