Add foreign_type option for polymorphic has_one and has_many.

To be possible to use a custom column name to save/read the polymorphic
associated type in a has_many or has_one polymorphic association, now users
can use the option :foreign_type to inform in what column the associated object
type will be saved.
Merge pull request #17970 from ulissesalmeida/foreign-type-has-many-has-one

Add foreign_type option for polymorphic has_one and has_many.