Fixed undefining of providers in src/{protocol,mixins}.lisp

* src/protocol.lisp (header): updated copyright
  (setf find-provider): adapted documentation string to fixed behavior
  (setf find-provider t symbol t): pass if-does-not-exist to next method
* src/mixins.lisp (header): updated copyright
  (setf find-provider eql nil povider-list-mixin symbol): fixed check
  for existence
* test/package.lisp (header): updated copyright
  (does-not-signal): new macro; ensure that a given condition is not
  signaled
* test/protocol.lisp (test protocol.find-provider.undefinition): new
  test; test signaled conditions and behavior when undefining an
  existing or non-existing provider