Bug 14098: FIX Copy a subfield should not update the original field

Bug 14098: FIX Copy a subfield should not update the original field

There is an inconsistency in the copy action:

Given the following control sample:

  245    _aThe art of computer programming
         _cDonald E. Knuth.
  300    _aA_exists
         _bB_exists

If we apply action (a) Copy the whole field 245 to 300, we get:

  245    _aThe art of computer programming
         _cDonald E. Knuth.
  300    _aA_exists
         _bB_exists
  300    _aThe art of computer programming
         _cDonald E. Knuth.

If we apply action (b) Copy the subfield 245$a to 300$a, we get:

  245    _aThe art of computer programming
         _cDonald E. Knuth.
  300    _aThe art of computer programming
         _bB_exists

In (a) the field is copied but in (b) the subfield is erased.

We should be consistent and don't erase the destination field.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>