Add option to make xml.tag! handle nil values like rails ActiveRecord::Base#to_xml

[hderms]

Nil values are handled differently in this gem versus rendering in the rails controller like:

render xml: @object

I think it would be nice if there was some way of instructing Builder to do something similar. More specifically, instead of a tag (foo) with a nil value being rendered like

<foo />

We could get something like:

<foo nil="true"/>

Having the option of representing nil values explicitly, without the user having to do the checks themselves would be nice.

[jimweirich]

Resolved in issue #34.