Return keypress when composing

Keypress should not happen during composing (as spec) but it does on Windows for Korean IMEs

src/input/ContentEditableInput.js

@@ -355,7 +355,7 @@ export default class ContentEditableInput {
   }
 
   onKeyPress(e) {
-    if (e.charCode == 0) return
+    if (e.charCode == 0 || this.composing) return
     e.preventDefault()
     if (!this.cm.isReadOnly())
       operation(this.cm, applyTextInput)(this.cm, String.fromCharCode(e.charCode == null ? e.keyCode : e.charCode), 0)