Allow nested attributes in associations to update values in it's owner object.  Fixes a regression from 3.0.x

This is to avoid confusing newbies, and to be consistent with the fact
that other options like :foreign_key already allow a symbol or a string.

Previously we would just silently write the attribute. This can lead to
subtle bugs (for example, see the change in AutosaveAssociation where a
through association would wrongly gain an attribute.

Also, ensuring that we never gain any new attributes after
initialization will allow me to reduce our dependence on method_missing.

Ensure we are not comparing a string with a symbol in HasManyAssociation#inverse_updates_counter_cache?. Fixes #2755, where a counter cache could be decremented twice as far as it was supposed to be.

Fixes #2832

The join_nodes must be passed to the JoinDependency initializer and therefore counted by the alias tracker. This is because the association_joins are aliased on initialization and then the tables are cached, so it is no use to alias the join_nodes later. Fixes #2556.

fix behavior when after_initialize is defined and a block is passed to Base.create

Assign the association attributes to the associated record before the before_initialize callback of the record runs. Fixes #1842.

The build_association method was added as an API for plugins
to hook into in 1398db01. This commit restores this API and the
ability to override class.new to return a subclass based on
a virtual attribute in the attributes hash.

CollectionAssociation#merge_target_lists should write to the underlying attributes when copying, rather than using the assignment method

Don't use mass-assignment protection when setting foreign keys or association conditions on singular associations. Fixes #481 (again).

Add test to specify that attributes from an association's conditions should be assigned without mass-assignment protection when a record is built on the association.