add invalid dmitri.shuralyov.com/test/modtest2 test module

This change adds a test module that is served over a module proxy
URL with an empty host (i.e., "https://"). It exists only for testing
purposes, since such module proxy URLs are not valid as of Go 1.13,
and all normal modules are being served with a module proxy URL that
has non-empty host as of commit 17791dc.

Updates #24
Updates golang/go#32006

test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"archive/zip"
 	"io"
 	"net/http"
 )
@@ -26,4 +27,49 @@ func init() {
 		http.HandleFunc("/test/modtest1/inner", h)
 		http.HandleFunc("/test/modtest1/inner/p", h)
 	}
+
+	// For testing Go module proxy URLs with an empty host. Such module proxy URLs unintentionally
+	// worked without error in Go 1.11 and 1.12. Go 1.13 has fixed it, so they no longer do.
+	// See https://golang.org/issue/32006#issuecomment-491943083.
+	{
+		http.HandleFunc("/test/modtest2", func(w http.ResponseWriter, req *http.Request) {
+			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+			io.WriteString(w, `<meta name="go-import" content="dmitri.shuralyov.com/test/modtest2 mod https://">`)
+		})
+		http.HandleFunc("/test/modtest2/@v/list", func(w http.ResponseWriter, req *http.Request) {
+			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+			io.WriteString(w, "v0.0.0\n")
+		})
+		http.HandleFunc("/test/modtest2/@v/v0.0.0.info", func(w http.ResponseWriter, req *http.Request) {
+			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+			io.WriteString(w, "{\n\t\"Version\": \"v0.0.0\",\n\t\"Time\": \"2019-05-04T15:44:36Z\"\n}\n")
+		})
+		http.HandleFunc("/test/modtest2/@v/v0.0.0.mod", func(w http.ResponseWriter, req *http.Request) {
+			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+			io.WriteString(w, "module dmitri.shuralyov.com/test/modtest2\n")
+		})
+		http.HandleFunc("/test/modtest2/@v/v0.0.0.zip", func(w http.ResponseWriter, req *http.Request) {
+			w.Header().Set("Content-Type", "application/zip")
+			z := zip.NewWriter(w)
+			for _, file := range []struct {
+				Name, Body string
+			}{
+				{"dmitri.shuralyov.com/test/modtest2@v0.0.0/go.mod", "module dmitri.shuralyov.com/test/modtest2\n"},
+				{"dmitri.shuralyov.com/test/modtest2@v0.0.0/p.go", "package p\n\n// Life is the answer.\nconst Life = 42\n"},
+			} {
+				f, err := z.Create(file.Name)
+				if err != nil {
+					panic(err)
+				}
+				_, err = f.Write([]byte(file.Body))
+				if err != nil {
+					panic(err)
+				}
+			}
+			err := z.Close()
+			if err != nil {
+				panic(err)
+			}
+		})
+	}
 }