fix: diffs are wrapped in :body when passed for formatting

[uldissturms]

The issue seems to be with diffs passed in a wrapped object {:body => diffs}, e.g.,

{:body=>[#<Pact::Matchers::Difference:0x007fb72e59cfa8 @expected="bar", @actual="WIFFLE", @message="Expected attribute bar but got WIFFLE at $.foo.thing.@name">]}

It's probably best to remove this knowledge from differ if possible and rely on array being passed in instead of a specific hash. Anyways, this seems to work when integrated with pact-ruby (pact-ruby-e2e-example).

Excerpt from logs:

# cat bar_mock_service.log
E, [2018-07-23T20:39:49.521404 #72173] ERROR -- : No matching interaction found for POST /thing
E, [2018-07-23T20:39:49.521453 #72173] ERROR -- : Interaction diffs for that route:
E, [2018-07-23T20:39:49.523228 #72173] ERROR -- : Diff with interaction: "a retrieve thing request"
EXPECTED : bar
ACTUAL   : WIFFLE
MESSAGE  : Expected attribute bar but got WIFFLE at $.foo.thing.@name

# cat bar_spec.rb
+ expect(bar_response.body).to eql ({})
expect(bar_response.status).to eql 201

Mock server response on error now resembles that of JSON:

{
  "message": "No interaction found for POST /thing",
  "interaction_diffs": [
    {
      "description": "a retrieve thing request",
      "body": [
        {
          "EXPECTED": "bar",
          "ACTUAL": "WIFFLE"
        }
      ]
    }
  ]
}

[bethesque]

I'm going to have to modify the diff response, because that response isn't very helpful is it!