Merge branch 'frozen-string-enable-lib-gitlab-gfm' into 'master'

Enable frozen string for spec/lib/gitlab/gfm

See merge request gitlab-org/gitlab!20248

spec/lib/gitlab/gfm/reference_rewriter_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Gfm::ReferenceRewriter do

spec/lib/gitlab/gfm/uploads_rewriter_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Gfm::UploadsRewriter do