Nil assignment returning old value when vault_lazy_decrypt! and vault_single_decrypt!

[kjakub]

After assigning nil to an encrypted attribute, the return value for the attribute is still the previous value.

example of failing test

it "allows dirty attributes to be unset" do
  person = LazySinglePerson.create!(ssn: "123-45-6789")
  person.ssn = nil
  expect(person.ssn).to be_nil
end

result

1. Vault::Rails lazy single decrypt allows dirty attributes to be unset
   Failure/Error: expect(person.ssn).to be_nil

   expected: nil
   got: "123-45-6789"
   ./spec/integration/rails_spec.rb:327:in `block (3 levels) in <top (required)>'