Compare responses with xml Content-Type

[ghost]

We need to compare xml responses, therefore I wrote a XmlLifter which simply converts the xml to json.

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[codecov-io]

Codecov Report

Merging #72 into master will decrease coverage by 0.6%.
The diff coverage is 5.88%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   35.04%   34.43%   -0.61%     
==========================================
  Files          29       30       +1     
  Lines         799      816      +17     
  Branches       34       34              
==========================================
+ Hits          280      281       +1     
- Misses        519      535      +16

Impacted Files	Coverage Δ
...ain/scala/com/twitter/diffy/lifter/XmlLifter.scala	0% <0%> (ø)
...in/scala/com/twitter/diffy/lifter/HttpLifter.scala	86.2% <11.11%> (-13.8%)	⬇️

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[puneetkhanduri]

Rather than create an XmlLifter here and adding a dependency on org.json4s, please use the existing jsoup dependency and use HtmlLifter to also lift xml.