Typing with Korean cannot be display properly in IE11

[sohee-lee7]

When I input using Korea in IE11, typing can't be display properly.

When typing using Korea in IE, the typing text is selected and collapsed of range is also false.
In this situation, Squire determines that one letter is selected and then remove the letter. In fact, the situation is in composition.

In this code, checking whether would perform the deletion logic.
https://github.com/neilj/Squire/blob/43b2c6b0e17a7fa1ce5b66c4fad326f3008a0846/source/KeyHandlers.js#L63-L73

Before this commit (b0ac7d3), key is empty string so the deletion logic was not performed.
But checking event.key occur this issue because IE does not support event.key. (https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key#Browser_compatibility)
In IE, the value of event.key is not undefined but is wrong value. So the length of event.key is 1 and the deletion logic performed. In additional, event.isComposing is not supported in IE.

I think before checking the length of event.key, need to check isIE.

[sohee-lee7]

When I test with checking isIE, typing Korean works well in IE.

--- a/source/KeyHandlers.js
+++ b/source/KeyHandlers.js
@@ -62,7 +62,7 @@ var onKey = function ( event ) {
     // !event.isComposing stops us from blatting Kana-Kanji conversion in Safari
     } else if ( !range.collapsed && !event.isComposing &&
             !event.ctrlKey && !event.metaKey &&
-            ( event.key || key ).length === 1 ) {
+            (isIE ? key : event.key || key).length === 1) {

But, I think this is not perfect solution. For more perfect, need to consider composition state or something for composition.
I wonder why this commit(b0ac7d3) was included and what problems there were.