Fix selective disabling of DateRangePicker

[majapw]

From @ajwild:

@majapw some refactoring of pull request #606 caused a problem here. If disabled is used directly then it causes an error because the individual date pickers still expect a boolean but they can end up with the string values. If you use !!disabled then the endDate string can disable the start date and vice versa. I've reverted this part back to checking equality with true. If you have a suggestion for a better way to handle this let me know.

Git went all weird for me, but this is a version of #1112 with regression tests.

Thanks @ajwild. Sorry for the weirdness. :/

@ljharb

[coveralls]

Coverage remained the same at 84.824% when pulling 13b733f on ajwild-fix into f8562f2 on master.

[ajwild]

@majapw I'd just done something similar. I also added checks for onFocusChange on DateRangePickerInputController_spec.js.

[majapw]

@ajwild if you share a commit with the controller spec you wrote I'll tack it on on top :)

[ajwild]

Try this one: ajwild@1bede26

[majapw]

Sweet, I'll merge this in once tests pass again. :)

[ajwild]

Thanks. I think #593 can be closed now 😄