Header extensibility processing model (Core, substantive)

Section 3.1, last paragraph states:

  'Note that each of the element information items described above
  allows attribute wildcards for future extensibility.'

We allow attribute extensibility but we don't describe a processing
model for it.  Consistent with EPR extensibility we suggest ignoring
extension attributes that you don't understand.

Append the following sentence to this paragraph:

  'Software processing these headers may safely ignore any extension
  attributes it does not recognize.'