Fixing the sequenced response stub configuration when data is in file

Hi Alexander,

The sequenced response works only when the stub response was in body, if the response were configured in file it was not handled. I have managed to fix it, please review and accept the pull request.

-  request:
      method: [GET]
      url: /uri/with/sequenced/responses/infile

   response:
      -  status: 201
         headers:
            content-type: application/json
         file: ../json/ok.json

      -  status: 201
         headers:
            content-stype: application/json
         file: ../json/goingstrong.json

      -  status: 500
         headers:
            content-type: application/json
         file: ../json/omfg.json