Fix devise specs for :devise_scope compatibility

[tagliala]

Devise 3.5.1 allows objects to specify their devise scope heartcombo/devise@4837bb0

So if we add allow_any_instance_of(User).to receive(:respond_to?).and_return(true) this will respond true here: https://github.com/plataformatec/devise/blob/4837bb0a4e5a1ea00e732f48ee2acef90606a31e/lib/devise/mapping.rb#L34

and the specs will fail.

Moreover, afaik, respond_to? should answer with true or false and this spec

    it 'does not cause error when email is nil' do
      allow_any_instance_of(User).to receive(:respond_to?).and_return(true)
      allow_any_instance_of(User).to receive(:respond_to?).with(:email).and_return(nil)

is doing something else. I think it was supposed to allow_any_instance_of(User).to receive(:email).and_return(nil)

[mshibuya]

Thanks for the fix!