Koenig - Prevent "Enter" being inserted into captions

refs TryGhost/Ghost#9724
- mobiledoc-kit's `key.isPrintable()` returns true for <kbd>Enter</kbd> but in this instance we don't want to capture newlines as printable chars
- swapped insertion of `event.key` for `key.toString()` for better handling of named keys that output characters (eg. when `event.key` === `Enter` which prints `\n`)

Author: kevinansfield

lib/koenig-editor/addon/components/koenig-caption-input.js

@@ -90,10 +90,10 @@ export default Component.extend({
         let key = new Key(event);
         let {editor} = this;
 
-        if (event.target.matches('[data-kg="editor"]') && editor && !editor._hasFocus() && key.isPrintableKey()) {
+        if (event.target.matches('[data-kg="editor"]') && editor && !editor._hasFocus() && key.isPrintableKey() && !key.isEnter()) {
             editor.focus();
             editor.run((postEditor) => {
-                postEditor.insertText(editor.post.tailPosition(), event.key);
+                postEditor.insertText(editor.post.tailPosition(), key.toString());
             });
 
             event.preventDefault();