http3: handle ErrAbortHandler when the handler panics (#3575)

* Handle abort like the stdlib does

* Using the sentinel value from the stdlib instead of redefining.

* we return before logging out

* Added test to hand abort handler

* Added in two tests but apparently only saved the first one.

* remove one test case because it wasn't needed

http3/server_test.go

@@ -201,6 +201,23 @@ var _ = Describe("Server", func() {
 			Expect(hfs).To(HaveKeyWithValue(":status", []string{"200"}))
 		})
 
+		It("handles a aborting handler", func() {
+			s.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				panic(http.ErrAbortHandler)
+			})
+
+			responseBuf := &bytes.Buffer{}
+			setRequest(encodeRequest(exampleGetRequest))
+			str.EXPECT().Context().Return(reqContext)
+			str.EXPECT().Write(gomock.Any()).DoAndReturn(responseBuf.Write).AnyTimes()
+			str.EXPECT().CancelRead(gomock.Any())
+
+			serr := s.handleRequest(conn, str, qpackDecoder, nil)
+			Expect(serr.err).ToNot(HaveOccurred())
+			hfs := decodeHeader(responseBuf)
+			Expect(hfs).To(HaveKeyWithValue(":status", []string{"500"}))
+		})
+
 		It("handles a panicking handler", func() {
 			s.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				panic("foobar")