Add support of nested structure to match body

[dpfg]

It should be possible to define body expectation using partial or fully copy of the payload.
E.g original payload

"repository": {
  "name": "bozr",
  "commits_num": 300,
  "creator": {
    "user_id": 8371394134,
    "name": "Author1"
  }
}

should pass test with the following expectation:

{
  "expect": {
    "body": {
        "repository": {
             "creator": {
                "user_id": 8371394134
             }
        }
    }
  }
}

Expectation is equal to

{
  "expect": {
    "body": {
      "repository.creator.user_id": 8371394134
    }
}