Set default for Hash#fetch w/ arg vs block in Manual#update
Set default for Hash#fetch w/ arg vs block in Manual#update

None of these defaults need to be in a block and so I think it's clearer
to use the two-argument version of `Hash#fetch`. This also makes the
code in `Manual#update` consistent with that in `Manual#initialize`.

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher