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 be also a
String value.
Merge pull request #14668 from laurocaetano/source_option_with_string_value

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