- Previously, If a ``BeforeRender`` event subscriber added a value via the
  ``__setitem__`` or ``update`` methods of the event object with a key that
  already existed in the renderer globals dictionary, a ``KeyError`` was
  raised.  With the deprecation of the "add_renderer_globals" feature of the
  configurator, there was no way to override an existing value in the
  renderer globals dictionary that already existed.  Now, the event object
  will overwrite an older value that is already in the globals dictionary
  when its ``__setitem__`` or ``update`` is called (as well as the new
  ``setdefault`` method), just like a plain old dictionary.  As a result, for
  maximum interoperability with other third-party subscribers, if you write
  an event subscriber meant to be used as a BeforeRender subscriber, your
  subscriber code will now need to (using ``.get`` or ``__contains__`` of the
  event object) ensure no value already exists in the renderer globals
  dictionary before setting an overriding value.