Add xml_encode function

[alexjfisher]

Example for unit test taken from
https://stackoverflow.com/questions/1443816/convert-an-arbitrary-string-to-xml-in-ruby

[puppet-community-rangefinder]

stdlib::xml_encode is a function

that may have no external impact to Forge modules.

This module is declared in 318 of 579 indexed public Puppetfiles.

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Exact matches are those that we can positively identify via namespace and the declaring modules' metadata. Non-namespaced items, such as Puppet 3.x functions, will always be reported as near matches only.

[david22swan]

@alexjfisher his change look's good to me, can't see anything wrong with it.
So gonna go ahead and merge, thanks for putting the work in :)