As previously implemented the `uid` and `gid` fields could only ever be
read as 0, because they were being initialized to zero and then
memoization (`@uid ||= uid`) was used to 'save' the new value. Using `nil`
as the initial value for either of these fields breaks so many tests, so I
have fixed this by not using memoization instead. This is safe because it
is only the local extra field that holds these values for this type of
extra field.

Remove some (almost) duplicated code and get ready for the real fix.

Previously, only the extra fields stored in the central directory were
being read in. In reality it is often the case that the extra field in
the central directory is just a marker, and the full data is in the
local header. So we need to read both in and merge the two into the
final correct extra field. This merging infrastructure was already
implemented in the extra field code but we never actually read the
local extra fields in until now.

Reading the central directory headers and local entry headers seems
rather fragile, so we can't just read one over the other and hope to end
up with a correctly merged set of extra fields because this breaks other
things. So we need to specifically read the local extra field data and
merge just those bits.

This commit also fixes a couple of tests that were 'broken' by us now
reading extra fields in correctly!

From what I can tell this was erroneously copied out of extra_field.rb
during a refactor. It attempts to compare a non-existent Hash that used
to be inherited before the refactor. If this code had been left within
ExtraField it would make more sense, but as it's not needed there either
let's just remove it.

See 20d79fe for the refactor.