parsing of second if it is an Expression object

[vheins]

array:5 [ // vendor/genealabs/laravel-model-caching/src/CacheKey.php:90
  "type" => "Column"
  "first" => "subscription_features.id"
  "operator" => "="
  "second" => Illuminate\Database\Query\Expression {#1951
    #value: ""subscription_plans"."feature_id""
  }
  "boolean" => "and"
]

@mikebronner please review & merge this