code: func HTTPBodyContains(t TestingT, handler http.HandlerFunc, method string, url string, values url.Values, str interface{}, msgAndArgs ...interface{}) {
  	if h, ok := t.(tHelper); ok {
  		h.Helper()
  	}
  	if assert.HTTPBodyContains(t, handler, method, url, values, str, msgAndArgs...) {
  		return
  	}
  	t.FailNow()
  }
docs: // HTTPBodyContains asserts that a specified handler returns a
  // body that contains a string.
  //
  //  assert.HTTPBodyContains(t, myHandler, "GET", "www.google.com", nil, "I'm Feeling Lucky")
  //
  // Returns whether the assertion was successful (true) or not (false).