code: @Override
      public void contains(JsonElement expectedJson) {
          assertTrue("Expected to find " + GSON.toJson(expectedJson), checkContains(expectedJson));
      }
docs: Asserts the actual response json payload contains to the expected json
  element, and writes that out to the output file. If this fails, the code will
  immediately exit, and record the error.
  
  @param expectedJson - the expected response json array