Turn on frozen string cop for ee/spec

Follow-up to:

https://gitlab.com/gitlab-org/gitlab/merge_requests/17754

.rubocop.yml

@@ -51,7 +51,6 @@ Style/FrozenStringLiteralComment:
     - 'danger/**/*'
     - 'db/**/*'
     - 'ee/db/**/*'
-    - 'ee/spec/**/*'
     - 'ee/lib/tasks/**/*'
     - 'lib/tasks/**/*'
     - 'qa/**/*'

ee/lib/csv_builder.rb

@@ -104,7 +104,7 @@ class CsvBuilder
   def excel_sanitize(line)
     return if line.nil?
 
-    line.prepend("'") if line =~ /^[=\+\-@;]/
+    line = ["'", line].join if line =~ /^[=\+\-@;]/
     line
   end
 end

ee/lib/gitlab/jira/middleware.rb

@@ -12,7 +12,9 @@ module Gitlab
       end
 
       def call(env)
-        env['HTTP_AUTHORIZATION']&.sub!('token', 'Bearer') if self.class.jira_dvcs_connector?(env)
+        if self.class.jira_dvcs_connector?(env)
+          env['HTTP_AUTHORIZATION'] = env['HTTP_AUTHORIZATION']&.sub('token', 'Bearer')
+        end
 
         @app.call(env)
       end

ee/spec/lib/csv_builder_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe CsvBuilder do

ee/spec/lib/gitlab/jira/middleware_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Jira::Middleware do