unescaping isn't needed for xml, fixes 'invalid %-encoding'

if we try to unescape an XML request that contains % (percent sign) it
makes Rack::Utils.unescape angry because it can't unescape single %.
also it's XML, decoding it like urlencoded request body is wrong.
Merge pull request #179 from everplays/ruby-invalid-character

(ruby) unescaping isn't needed for xml, fixes 'invalid %-encoding'