Don't override headers and delete methods in AR4

@maartenvg @pickle27 

Neither of the overrides in `base_ext` are necessary in AR4.

[AR4's version of `headers`](https://github.com/rails/activeresource/blob/v4.0.0/lib/active_resource/base.rb#L603) is the same as is defined here, as is [the code for `delete`](https://github.com/rails/activeresource/blob/v4.0.0/lib/active_resource/base.rb#L922)