Revisiting Age error handling

[mnot]

My tests for Age parsing seem to indicate we might want to reconsider how errors are handled.

Of implementations that honour Age (two don't):

• none will consider 0, 0 invalid
• none will consider 0 on two separate field lines invalid
• none will consider a duplicated value on two lines to make the header invalid
• if there are two Age header lines, all will consider only the first (and one, Safari, appears to use the min of them)
• none considers a non-numeric value to be invalid
• only Fastly and Firefox consider a non-negative integer to be invalid

... where invalid currently is required to be considered stale.