Rails 3.2 write_attribute fix

Updating to Rails 3.2 causes failure when injecting the tenant association on new objects.  The attribute isn't there yet, and fails.  The only attributes that are available at the time are the database columns themselves, so instead of using object.account, I used object.account_id and all is well.

Possibly related: https://github.com/rails/rails/issues/4583

Guessing it's this commit: https://github.com/rails/rails/commit/50d395f96ea05da1e02459688e94bff5872c307b

Full stack trace of the failure:

```
/Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/attribute_methods/write.rb:34:in `write_attribute': can't write unknown attribute `account' (ActiveModel::MissingAttributeError)
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/attribute_methods/dirty.rb:67:in `write_attribute'
	from /Volumes/Aramis/acts_as_tenant.git/lib/acts_as_tenant/model_extensions.rb:62:in `block in acts_as_tenant'
	from /Volumes/Aramis/acts_as_tenant.git/lib/acts_as_tenant/model_extensions.rb:43:in `block in acts_as_tenant'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activesupport-3.2.1/lib/active_support/callbacks.rb:407:in `_run__2098072948231748962__validation__677429581214301363__callbacks'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activesupport-3.2.1/lib/active_support/callbacks.rb:405:in `__run_callback'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activesupport-3.2.1/lib/active_support/callbacks.rb:385:in `_run_validation_callbacks'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activesupport-3.2.1/lib/active_support/callbacks.rb:81:in `run_callbacks'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activemodel-3.2.1/lib/active_model/validations/callbacks.rb:53:in `run_validations!'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activemodel-3.2.1/lib/active_model/validations.rb:179:in `valid?'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/validations.rb:69:in `valid?'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/validations.rb:77:in `perform_validations'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/validations.rb:50:in `save'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/attribute_methods/dirty.rb:22:in `save'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:241:in `block (2 levels) in save'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:295:in `block in with_transaction_returning_status'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/connection_adapters/abstract/database_statements.rb:190:in `transaction'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:208:in `transaction'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:293:in `with_transaction_returning_status'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:241:in `block in save'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:252:in `rollback_active_record_state!'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/transactions.rb:240:in `save'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/activerecord-3.2.1/lib/active_record/persistence.rb:45:in `create'
	from /Volumes/Aramis/acts_as_tenant.git/spec/model_extensions_spec.rb:221:in `block (2 levels) in <top (required)>'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/example_group.rb:201:in `module_eval'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/example_group.rb:201:in `subclass'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/example_group.rb:187:in `describe'
	from /Volumes/Aramis/acts_as_tenant.git/spec/model_extensions_spec.rb:218:in `block in <top (required)>'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/example_group.rb:201:in `module_eval'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/example_group.rb:201:in `subclass'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/example_group.rb:187:in `describe'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/dsl.rb:18:in `describe'
	from /Volumes/Aramis/acts_as_tenant.git/spec/model_extensions_spec.rb:78:in `<top (required)>'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/configuration.rb:698:in `load'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/configuration.rb:698:in `block in load_spec_files'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/configuration.rb:698:in `map'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/configuration.rb:698:in `load_spec_files'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/command_line.rb:22:in `run'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/runner.rb:80:in `run_in_process'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/runner.rb:69:in `run'
	from /Users/nick/.rvm/gems/ruby-1.9.2-p290@rails31/gems/rspec-core-2.8.0/lib/rspec/core/runner.rb:10:in `block in autorun'
```