Makes it consistent with Relation. Can't see a use for this.

This helps bring the interfaces of CollectionProxy and Relation closer
together, and reduces the delegation backflips we need to perform.

For example, first_or_create is defined thus:

class ActiveRecord::Relation
  def first_or_create(...)
    first || create(...)
  end
end

If CollectionProxy < Relation, then post.comments.first_or_create will
hit the association's #create method which will actually add the new record
to the association, just as post.comments.create would.

With the previous delegation, post.comments.first_or_create expands to
post.comments.scoped.first_or_create, where post.comments.scoped has no
knowledge of the association.

Change CollectionProxy#method_missing to use scoped.public_send, to
avoid a problem described in issue #2508 when trying to use class
methods with names like "open", that clash with private kernel methods.
Also changed the dynamic matcher instantiator to send straight to
scoped, to avoid another roundtrip to method_missing.

if they are not defined or delegated.
In this way, we have a performance boost invoking some array methods
which are not defined in CollectionAssociation.

CollectionProxy#replace should change the DB records rather than just mutating the array. Fixes #3020.

Add a proxy_association method to association proxies, which can be called by association extensions to access information about the association. This replaces proxy_owner etc with proxy_association.owner.

Add a proxy_association method to association proxies, which can be called by association extensions to access information about the association. This replaces proxy_owner etc with proxy_association.owner.

added an alias for new to build to the AR collection proxy, this corrects an issue where the collection proxies were not consistent