The 'SpaceHelmet' fairing is now called 'Shield'. It features the
following changes and improvements:
* Headers which are now ignored by browsers are removed.
* 'XssFilter' is no longer an on-by-default policy.
* A new 'Permission' policy is introduced.
* 'Shield' is attached to all 'Rocket' instances by default.
* Default headers never allocate on 'Clone'.
* Policy headers are rendered once and cached at start-up.
* Improved use of typed URIs in policy types.
Sergio Benitez