Avoid infinite recursive validation when validate => true

An outgrowth of #2144 Don't autosave new_record associations. Since the solution to my problem was apparently "validate => true". This happens to be a problem for my situation.

I happen to be in a situation like this:

  def test_do_the_right_thing_on_validate
    Pirate.class_eval do
      belongs_to :parrot, :validate => true
    end
    Parrot.class_eval do
      has_and_belongs_to_many :pirates, :validate => true
    end
    
    pirate = Pirate.new(:catchphrase => "Figure out how to validate me please")
    pirate.parrot = Parrot.new
    pirate.parrot.pirates << pirate

    #because the parrot doesn't have a name, this save should fail invalid
    assert_raises(ActiveRecord::RecordInvalid){
      pirate.save!      
    }
  end

And, as you might have guessed, if you run this test....

  1) Failure:
test_do_the_right_thing_on_validate(TestStuff)
    [./test/cases/autosave_association_test.rb:883:in `test_do_the_right_thing_on_validate'
     ./test/cases/../../../activesupport/lib/active_support/testing/setup_and_teardown.rb:57:in `__send__'
     ./test/cases/../../../activesupport/lib/active_support/testing/setup_and_teardown.rb:57:in `run']:
<ActiveRecord::RecordInvalid> exception expected but was
Class: <SystemStackError>
Message: <"stack level too deep">
---Backtrace---
./test/cases/../../lib/active_record/callbacks.rb:312:in `valid?'
./test/cases/../../lib/active_record/associations/association_proxy.rb:221:in `send'
./test/cases/../../lib/active_record/associations/association_proxy.rb:221:in `method_missing'
./test/cases/../../lib/active_record/autosave_association.rb:249:in `association_valid?'
./test/cases/../../lib/active_record/autosave_association.rb:229:in `validate_single_association'
./test/cases/../../lib/active_record/autosave_association.rb:183:in `validate_associated_records_for_parrot'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:180:in `evaluate_method'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:180:in `evaluate_method'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:180:in `instance_eval'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:180:in `evaluate_method'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:166:in `call'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:90:in `run'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:90:in `each'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:90:in `send'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:90:in `run'
./test/cases/../../../activesupport/lib/active_support/callbacks.rb:276:in `run_callbacks'
./test/cases/../../lib/active_record/validations.rb:1029:in `valid_without_callbacks?'
./test/cases/../../lib/active_record/callbacks.rb:315:in `valid?'
...
./test/cases/../../lib/active_record/callbacks.rb:315:in `valid?'
./test/cases/../../lib/active_record/validations.rb:1018:in `save_without_dirty!'
./test/cases/../../lib/active_record/dirty.rb:87:in `save_without_transactions!'
./test/cases/../../lib/active_record/transactions.rb:200:in `save!'
./test/cases/../../lib/active_record/connection_adapters/abstract/database_statements.rb:136:in `transaction'
./test/cases/../../lib/active_record/transactions.rb:182:in `transaction'
./test/cases/../../lib/active_record/transactions.rb:200:in `save!'
./test/cases/../../lib/active_record/transactions.rb:208:in `rollback_active_record_state!'
./test/cases/../../lib/active_record/transactions.rb:200:in `save!'
./test/cases/autosave_association_test.rb:884:in `test_do_the_right_thing_on_validate'
./test/cases/autosave_association_test.rb:883:in `test_do_the_right_thing_on_validate'
./test/cases/../../../activesupport/lib/active_support/testing/setup_and_teardown.rb:57:in `__send__'
./test/cases/../../../activesupport/lib/active_support/testing/setup_and_teardown.rb:57:in `run'
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