Improve spec branch coverage for ConfigFormatter

`AMENDMENTS` was defined as a String, not an Array. And splatting the
array into `#gsub` doesn't work if there is more than one element - we
need to use `Regexp.union` instead.

lib/rubocop/rspec/config_formatter.rb

@@ -7,7 +7,7 @@ module RSpec
     # Builds a YAML config file from two config hashes
     class ConfigFormatter
       EXTENSION_ROOT_DEPARTMENT = %r{^(RSpec/)}.freeze
-      AMENDMENTS = %(Metrics/BlockLength)
+      AMENDMENTS = %w[Metrics/BlockLength].freeze
       COP_DOC_BASE_URL = 'https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/'
 
       def initialize(config, descriptions)
@@ -17,8 +17,8 @@ def initialize(config, descriptions)
 
       def dump
         YAML.dump(unified_config)
-          .gsub(EXTENSION_ROOT_DEPARTMENT, "\n\\1")
-          .gsub(*AMENDMENTS, "\n\\0")
+          .gsub(EXTENSION_ROOT_DEPARTMENT, "\n\\0")
+          .gsub(Regexp.union(AMENDMENTS), "\n\\0")
           .gsub(/^(\s+)- /, '\1  - ')
           .gsub('"~"', '~')
       end
@@ -27,8 +27,6 @@ def dump
 
       def unified_config
         cops.each_with_object(config.dup) do |cop, unified|
-          next if AMENDMENTS.include?(cop)
-
           replace_nil(unified[cop])
           unified[cop].merge!(descriptions.fetch(cop))
           unified[cop]['Reference'] = reference(cop)

spec/rubocop/rspec/config_formatter_spec.rb

@@ -8,6 +8,12 @@
       'AllCops' => {
         'Setting' => 'forty two'
       },
+      'Metrics/BlockLength' => {
+        'Exclude' => [
+          '**/*_spec.rb',
+          '**/spec/**/*'
+        ]
+      },
       'RSpec/Foo' => {
         'Config' => 2,
         'Enabled' => true
@@ -45,6 +51,11 @@
       AllCops:
         Setting: forty two
 
+      Metrics/BlockLength:
+        Exclude:
+          - "**/*_spec.rb"
+          - "**/spec/**/*"
+
       RSpec/Foo:
         Config: 2
         Enabled: true