Default to auto height being true.

Otherwise TextEditorElements created through the tag wouldn’t have a
setting and be wrong.

src/text-editor-element.coffee

@@ -18,6 +18,7 @@ class TextEditorElement extends HTMLElement
   hasTiledRendering: true
   logicalDisplayBuffer: true
   scrollPastEnd: true
+  autoHeight: true
 
   createdCallback: ->
     # Use globals when the following instance variables aren't set.