lambda support for XML -> Ruby conversion

[yob]

For reference, the Product class in my ONIX library: http://github.com/yob/onix/blob/master/lib/onix/product.rb, lines 31-37.

The ONIX spec requires all dates to be formatted as YYYYMMDD.

I handle the to_xml conversion from a Date class to a string by defining a lambda method and passing it to the attribute using the :to_xml option.

For marshalling xml strings into Date objects, I was using ":as => Date", but that would raise an exception when I tried to parse a file that containing invalid dates ( like "00000000"). To define my own XML->Ruby process, I have to pass a block into the attribute definition. This works, but has lead to some duplication.

It would be awesome if I could pass a lambda object into :as, just like I can with :to_xml.