Mark Rails/UniqBeforePluck cops as unsafe

`Rails/UniqBeforePluck` is unsafe depending on the database collation.
Here is an example of an unsafe case for the three databases that Rails supports(MySQL, SQLite, PostgreSQL).

### In the case of MySQL

The default collation is `utf8mb4_general_ci`(in Rails), so it is unsafe.

An example is as follows.
- Create table
```
class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name
    end
  end
end
```

- Create data
```
User.create!(name: 'A')
User.create!(name: 'a')
```

before autocorrect
```
User.pluck(:name).uniq #=> ["A", "a"]
```

after autocorrect
```
User.distinct.pluck(:name) #=> ["A"]
```

### In the case of SQLite

The default collation is `BINARY`, so it is safe.
If you change the collation to `NOCASE`, the collation is unsafe.

The following is an example.

- Create table
```
class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name, collation: 'NOCASE'
    end
  end
end
```

Other than the above, it is the same as MySQL.

### In the case of PostgreSQL

The default collation is safe.
If you create a custom collation and specify it, the collation will be unsafe.

A concrete example is as follows.

- Create custom collation
```
CREATE COLLATION case_insensitive (
  provider = icu,
  locale = 'und-u-ks-level2',
  deterministic = false
);
```

- Create table
```
class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name, collation: 'case_insensitive'
    end
  end
end
```

Other than the above, it is the same as MySQL.

CHANGELOG.md

@@ -18,6 +18,7 @@
 * [#312](https://github.com/rubocop-hq/rubocop-rails/pull/312): Mark `Rails/MailerName` as unsafe for auto-correct. ([@eugeneius][])
 * [#294](https://github.com/rubocop-hq/rubocop-rails/pull/294): Update `Rails/ReversibleMigration` to register offenses for `remove_columns` and `remove_index`. ([@philcoggins][])
 * [#310](https://github.com/rubocop-hq/rubocop-rails/issues/310): Add `EnforcedStyle` to `Rails/PluckInWhere`. By default, it does not register an offense if `pluck` method's receiver is a variable. ([@koic][])
+* [#320](https://github.com/rubocop-hq/rubocop-rails/pull/320): Mark `Rails/UniqBeforePluck` as unsafe. ([@kunitoo][])
 
 ## 2.7.1 (2020-07-26)
 
@@ -262,3 +263,4 @@
 [@taylorthurlow]: https://github.com/taylorthurlow
 [@krim]: https://github.com/krim
 [@philcoggins]: https://github.com/philcoggins
+[@kunitoo]: https://github.com/kunitoo

config/default.yml

@@ -657,11 +657,12 @@ Rails/UniqBeforePluck:
   Description: 'Prefer the use of uniq or distinct before pluck.'
   Enabled: true
   VersionAdded: '0.40'
-  VersionChanged: '2.6'
+  VersionChanged: '2.8'
   EnforcedStyle: conservative
   SupportedStyles:
     - conservative
     - aggressive
+  Safe: false
   AutoCorrect: false
 
 Rails/UniqueValidationWithoutIndex:

docs/modules/ROOT/pages/cops_rails.adoc

@@ -3977,10 +3977,10 @@ Time.at(timestamp).in_time_zone
 | Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
 
 | Enabled
-| Yes
-| Yes
+| No
+| Yes (Unsafe)
 | 0.40
-| 2.6
+| 2.8
 |===
 
 Prefer the use of distinct, before pluck instead of after.
@@ -3998,6 +3998,8 @@ as the cop cannot distinguish between calls to pluck on an
 ActiveRecord::Relation vs a call to pluck on an
 ActiveRecord::Associations::CollectionProxy.
 
+This cop is unsafe because the behavior may change depending on the
+database collation.
 Autocorrect is disabled by default for this cop since it may generate
 false positives.
 

lib/rubocop/cop/rails/uniq_before_pluck.rb

@@ -18,6 +18,8 @@ module Rails
       # ActiveRecord::Relation vs a call to pluck on an
       # ActiveRecord::Associations::CollectionProxy.
       #
+      # This cop is unsafe because the behavior may change depending on the
+      # database collation.
       # Autocorrect is disabled by default for this cop since it may generate
       # false positives.
       #