Handle private collection accessor with have(n).items

`expect(owner).to have(n).things` invokes private owner#things with
Object#__send__ if the owner does not respond to any of #size, #count
and #length.

https://github.com/rspec/rspec-expectations/blob/v2.14.3/lib/rspec/matchers/built_in/have.rb#L48-L58

This is related to #5.
Use absolute character positions as node ID in dynamic analysis