Feature/Update base serializer attributes

Added ability to show not_rfc_mx_lookup_flow attribute in serialized validation result:

Truemail.validate('nonexistent_email@bestweb.com.ua').as_json

=>
# Serialized Truemail::Validator instance
{
  "date": "2020-05-10 10:00:00 +0200",
  "email": "nonexistent_email@bestweb.com.ua",
  "validation_type": "smtp",
  "success": false,
  "errors": {
    "smtp": "smtp error"
  },
  "smtp_debug": [
    {
      "mail_host": "213.180.193.89",
      "port_opened": true,
      "connection": true,
      "errors": {
        "rcptto": "550 5.7.1 No such user!\n"
      }
    }
  ],
  "configuration": {
    "validation_type_by_domain": null,
    "whitelist_validation": false,
    "whitelisted_domains": null,
    "blacklisted_domains": null,
    "not_rfc_mx_lookup_flow": false,
    "smtp_safe_check": false,
    "email_pattern": "default gem value",
    "smtp_error_body_pattern": "default gem value"
  }
}

1. Updated Truemail::Log::Serializer::Base
2. Updated gem documentation
3. Updated gem version to 1.7.1