Add ability to turn of collaping empty elements

[ser]

Currently, when printing, REXML collapses empty elements:

  puts REXML::Document.new( "<a></a>" )

produces:

  <a/>

We want to be able to turn this off, so that there are no collapsed elements:

  puts REXML::Document.new("<a/>", :collapse=>false)
  # => "<a></a>"

[ser]

Pulling into 3.1.8