Fix spec:appsec:rack for Rack 3.1 (latest)

Matrixfile

@@ -258,7 +258,7 @@
     'redis-5' => '✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby'
   },
   'appsec:rack' => {
-    # 'rack-latest' => '✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby',
+    'rack-latest' => '✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby',
     'rack-3'      => '✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby',
     'rack-2'      => '✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby',
     # Non-deprecated form of Regexp.new does not backport to Rack 1.x, see: https://github.com/rack/rack/pull/1998

lib/datadog/appsec/contrib/rack/gateway/request.rb

@@ -45,14 +45,11 @@ def headers
               end
 
               result['content-type'] = request.content_type if request.content_type
-              result['content-length'] = request.content_length if request.content_length
+              # Since Rack 3.1, content-length is nil if the body is empty, but we still want to send it to the WAF.
+              result['content-length'] = request.content_length || '0'
               result
             end
 
-            def body
-              request.body.read.tap { request.body.rewind }
-            end
-
             def url
               request.url
             end

spec/datadog/appsec/contrib/rack/gateway/request_spec.rb

@@ -65,12 +65,6 @@
     end
   end
 
-  describe '#body' do
-    it 'returns the body' do
-      expect(request.body).to eq('')
-    end
-  end
-
   describe '#url' do
     it 'returns the url' do
       expect(request.url).to eq('http://example.com:8080/?a=foo&a=bar&b=baz')