Add regression coverage for remeasuring characters w/ requested update

The problem here is that the lines in the editor no longer reflect the
lines on screen, but we're looking for line nodes corresponding to the
editor's current contents.

spec/editor-component-spec.coffee

@@ -1953,6 +1953,7 @@ describe "EditorComponent", ->
         wrapperView.hide()
 
         component.setFontSize(22)
+        editor.getBuffer().insert([0, 0], 'a') # regression test against atom/atom#3318
 
         wrapperView.show()
         editor.setCursorBufferPosition([0, Infinity])