Fixes save behaviour for RepresentativeForm (fixes #261)

When RepresentaiveForm#has_representative? is false:
  - Do not create representative relation on claim
  - Destroy relation is exists

When RepresentaiveForm#has_representative? is true:
  - Create representative relation on claim
  - Do not destroy relation is exists
Merge pull request #262 from ministryofjustice/bug-fix/do-not-create-representative-if-user-chooses-no

Fixes save behaviour for RepresentativeForm (fixes #261)