Fix hbtm reflection

This adds back `has_and_belongs_to_many` reflection to the reflections
hash. Even thought we are not using that internally, as
`Model.reflections` is a public method we need to keep the hash of
reflections coherent.

ad7b5ef is the commit that made
`has_and_belongs_to_many became` a `has_many :through` and a `has_many`.

review @tenderlove @rafaelfranca @matthewd

I am still not 100% convinced we need to do this, but I guess thats what was proposed on the issue.

[fixes #14682]