Add tests for setting trusted_proxies

Each test reloads the trusted_proxies initializer, which in turn will set Rails.application.config.action_dispatch.trusted_proxies to something new. This will leak into the other tests, but the middleware that it is used in has already been loaded for the whole test suite, so it should have no impact.

spec/initializers/trusted_proxies_spec.rb

+require 'spec_helper'
+
+describe 'trusted_proxies', lib: true do
+  context 'with default config' do
+    before do
+      set_trusted_proxies([])
+    end
+
+    it 'preserves private IPs as remote_ip' do
+      request = stub_request('HTTP_X_FORWARDED_FOR' => '10.1.5.89')
+      expect(request.remote_ip).to eq('10.1.5.89')
+    end
+
+    it 'filters out localhost from remote_ip' do
+      request = stub_request('HTTP_X_FORWARDED_FOR' => '1.1.1.1, 10.1.5.89, 127.0.0.1')
+      expect(request.remote_ip).to eq('10.1.5.89')
+    end
+  end
+
+  context 'with private IP ranges added' do
+    before do
+      set_trusted_proxies([ "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16" ])
+    end
+
+    it 'filters out private and local IPs from remote_ip' do
+      request = stub_request('HTTP_X_FORWARDED_FOR' => '1.2.3.6, 1.1.1.1, 10.1.5.89, 127.0.0.1')
+      expect(request.remote_ip).to eq('1.1.1.1')
+    end
+  end
+
+  context 'with proxy IP added' do
+    before do
+      set_trusted_proxies([ "60.98.25.47" ])
+    end
+
+    it 'filters out proxy IP from remote_ip' do
+      request = stub_request('HTTP_X_FORWARDED_FOR' => '1.2.3.6, 1.1.1.1, 60.98.25.47, 127.0.0.1')
+      expect(request.remote_ip).to eq('1.1.1.1')
+    end
+  end
+
+  def stub_request(headers = {})
+    ActionDispatch::RemoteIp.new(Proc.new { }, false, Rails.application.config.action_dispatch.trusted_proxies).call(headers)
+    ActionDispatch::Request.new(headers)
+  end
+
+  def set_trusted_proxies(proxies = [])
+    stub_config_setting('trusted_proxies' => proxies)
+    load File.join(__dir__, '../../config/initializers/trusted_proxies.rb')
+  end
+end