Correctly output boolean element value 'false'

The boolean value `false` is a valid value for a Boolean element and should be rendered as any other value. However, when determining whether an element value should be included in the output of `to_xml`, the check is `elsif item`, which fails when `item == false`. As a result a Boolean element with value `false` currently is not included in the `to_xml` output. This patch fixes that.
Merge pull request #44 from Confusion/patch-1

Correctly output boolean element value 'false'