Fixed case insensitive uniqueness validation for localized fields

Localized fields should be validated separately for each localization because the value should be unique in each locale. 

For example, we have a document with localized attribute:

    { description: { "en" => "Car", "de" => "Auto" } }

We want to add a new document: 

    { description: { "en" => "Motorbike", "de" => "Auto" } }

The description hash is different but we have the same value for "de" locale. To solve this issue I implemented uniqueness validation for licalized fields using $or selection:

    validatable.or(
      { "description.en" => "Motorbike" }, { "description.de" => "Auto" }
    )

case_sensitive flag was also fixed.