MouseUp manual `blur()`s close day picker in IE

[bdrazen]

react-dates version
e.g. react-dates@18.3.1

Describe the bug
Manual blur triggers on MouseUp cause the day picker to close in IE. Can be reproduced by changing the month or clicking the keyboard shortcuts icon.

Is the issue reproducible in Storybook?
http://airbnb.io/react-dates/?selectedKind=SingleDatePicker%20%28SDP%29&selectedStory=default&full=0&addons=1&stories=1&panelRight=0&addonPanel=storybook%2Factions%2Factions-panel

Additional context
Commenting out the .blur() lines in the following files prevents the behaviour:

src/components/DayPicker.jsx
src/components/DayPickerKeyboardShortcuts.jsx
src/components/DayPickerNavigation.jsx