The :source option for has_many => through should accept String values.

With the changes introduced by 16b70fddd4dc7e7fb7be108add88bae6e3c2509b it was expecting the value to be a Symbol, while it could also be a String value.

The solution was call `to_sym` on the given `:source`.

Fixes https://github.com/rails/rails/issues/14651

This PR should only be backported to `4-1-stable`