Fix dirty nil assignments

[chrisarcand]

Description

Fixes a regression caused by 6c5b1a3

The usage of instance_variable_get means that if an attribute is assigned to nil, the variable to signal that attributes are loaded is false - meaning that any intentional setting of nil results in attributes getting reloaded on the next read and wiping that nil value back to whatever it was before. I've added some specs to describe this behavior.

[Valarissa]

Great catch! This looks great.

[radditude]

Nice!

[gdemonie]

This doesn't seem to fix the case where vault_single_decrypt! is enabled?
After assigning nil to an encrypted attribute, the return value for the attribute is still the previous value.
There is a test for that case with LazyPerson:

  it "allows dirty attributes to be unset" do
      person = LazyPerson.create!(ssn: "123-45-6789")
      person.ssn = nil
      expect(person.ssn).to be_nil
      person2 = LazyPerson.create!(ssn: "123-45-6789")
      person2.assign_attributes(ssn: nil)
      expect(person2.ssn).to be_nil
    end

The same case with LazySinglePerson doesn't have the same test.