Explicitly set the case_sensitive option for uniqueness validators

On Rails 6 we see the following deprecation:

    DEPRECATION WARNING: Uniqueness validator will no longer enforce
    case sensitive comparison in Rails 6.1. To continue case sensitive
    comparison on the :name attribute in ActsAsTaggableOn::Tag model,
    pass `case_sensitive: true` option explicitly to the uniqueness
    validator.

Also update the validation syntax to the "new" and recommended way
introduced in Rails 4.

CHANGELOG.md

@@ -13,7 +13,8 @@ As such, _Breaking Changes_ are major. _Features_ would map to either major or m
 ### [Master / Unreleased](https://github.com/mbleigh/acts-as-taggable-on/compare/v6.5.0...master)
 
 * Fixes
-  * [@nbulaj Add support for Ruby 2.7 and it's kwargs](https://github.com/mbleigh/acts-as-taggable-on/pull/910)  
+  * [@nbulaj Add support for Ruby 2.7 and it's kwargs](https://github.com/mbleigh/acts-as-taggable-on/pull/910)
+  * [@FunnyHector Fix the uniqueness validator deprecation warning on Rails 6](https://github.com/mbleigh/acts-as-taggable-on/pull/1009)
 
 ### [6.5.0 / 2019-11-07](https://github.com/mbleigh/acts-as-taggable-on/compare/v6.0.0...v6.5.0)
 

lib/acts_as_taggable_on/tag.rb

@@ -9,9 +9,10 @@ class Tag < ::ActiveRecord::Base
 
     ### VALIDATIONS:
 
-    validates_presence_of :name
-    validates_uniqueness_of :name, if: :validates_name_uniqueness?
-    validates_length_of :name, maximum: 255
+    validates :name,
+              presence: true,
+              uniqueness: { case_sensitive: ActsAsTaggableOn.strict_case_match, if: :validates_name_uniqueness? },
+              length: { maximum: 255 }
 
     # monkey patch this method if don't need name uniqueness validation
     def validates_name_uniqueness?