DEV: fix rubocop errors

discourse_mail_receiver.gemspec

@@ -2,7 +2,7 @@
 
 # coding: utf-8
 lib = File.expand_path('../lib', __FILE__)
-$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+$LOAD_PATH.unshift(lib) if !$LOAD_PATH.include?(lib)
 
 Gem::Specification.new do |spec|
   spec.name          = 'discourse_mail_receiver'

lib/mail_receiver/fast_rejection.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-require 'set'
 require 'syslog'
 require 'json'
 require 'uri'

lib/mail_receiver/mail_receiver_base.rb

@@ -5,7 +5,7 @@ class ReceiverException < StandardError; end
   attr_reader :env
 
   def initialize(env_file)
-    unless File.exists?(env_file)
+    unless File.exist?(env_file)
       fatal "Config file %s does not exist. Aborting.", env_file
     end
 

spec/lib/fast_reject_spec.rb → spec/lib/fast_rejection_spec.rb

@@ -23,7 +23,7 @@
     expect(receiver.endpoint).to eq('https://localhost:8080/admin/email/smtp_should_reject.json')
   end
 
-  context "process_single_request" do
+  describe "#process_single_request" do
     let(:receiver) { described_class.new(file_for(:standard)) }
 
     it "returns defer_if_permit if not smtpd_access_policy" do
@@ -145,5 +145,4 @@
       expect(response).to eq("reject because I said so")
     end
   end
-
 end

spec/lib/mail_spec.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 require_relative '../../lib/mail_receiver/mail'
 
+# rubocop:disable RSpec/DescribeClass
+# mail.rb is not implemented as a class or module
 RSpec.describe 'domain_from_addrspec' do
 
   it "normalises domains to lowercase" do