BibField: fix copyright field names

* Fixes a typo for one of the copyright fields and also changes its name
  to `copyright_information` to avoid duplicate names. (closes #1933)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
You may perhaps name the field `copyright_status` as this is how MARC standard puts it. http://www.loc.gov/marc/bibliographic/bd542.html