Revisit reshaping example possibly outdated by fread recognizing dates

[MichaelChirico]

          Actually this is an interesting point, this example was designed for when `dob_child1` etc. would be read by `fread()` as type `character`, but now they're recognized as dates and read as `IDate`.

Before this example illustrated gender being cast from integer to character, now it only goes integer->integer, but gains the class attribute. Maybe we should revisit this.

Originally posted by @MichaelChirico in #6521 (comment)