Ensure fixture context always contains the key template_inject with a dict value

[laundmo]

This means various fixtures won't have to check if it exists.

Solves the issue that Auth overwrites the entire template_inject dict when Auth.inject is true.

follow-up/related ideas:

• Unify the various things being added to the render context (request, HELPERS, URL), by adding them to template_inject from the start
• Allow a way to define template helper functions without having to Inject() in each controller, using a decorator which stores them in a global variable added to template_inject like this

  @template_helper
  def myhelper():
      return "test"