Bug 13437: Preliminary changes for marc21 plugins field 008

Bug 13437: Preliminary changes for marc21 plugins field 008

Preliminary work on marc21_field_008.pl and 008_authorities.pl.
Moving $dateentered en $defaultval to lower scope level.
date_entered is a new sub (in a module); sub Field008 has been reduced to
one line.
Added a trivial unit test for date_entered.
Note: the format used in date_entered could be added in DateUtils, moving
this logic to a better place.

Test plan:
Use both plugins in the marc21 editor (biblios/authorities).
Run the adjusted unit test.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Both plugins works, no errors

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>