Return default value of true from TextEditor.getAutoHeight

When we need to check for unassigned in the deprecated code path, we
can just read the instance variable directly to avoid getting the
deafult.

Signed-off-by: Antonio Scandurra <as-cii@github.com>

src/text-editor.coffee

@@ -3542,7 +3542,7 @@ class TextEditor extends Model
     Grim.deprecate("This is now a view method. Call TextEditorElement::getHeight instead.")
     @height
 
-  getAutoHeight: -> @autoHeight
+  getAutoHeight: -> @autoHeight ? true
 
   setWidth: (width, reentrant=false) ->
     if reentrant