perf: clone request URL struct directly for path manipulation

Save 4 allocations worth 200B per request cycle with redirect.
The rewrite operation takes before O(600ns) vs after O(200ns).

```
$ go test -benchmem -benchtime 5000000x -bench BenchmarkStrictSlash
goos: linux
goarch: amd64
pkg: github.com/gorilla/mux
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
BenchmarkStrictSlashClone-8      5000000               183.9 ns/op            64 B/op          4 allocs/op
BenchmarkStrictSlashParse-8      5000000               559.8 ns/op           264 B/op          8 allocs/op
PASS
ok      github.com/gorilla/mux  3.740s

$ go test -benchmem -benchtime 5000000x -bench BenchmarkStrictSlash
goos: linux
goarch: amd64
pkg: github.com/gorilla/mux
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
BenchmarkStrictSlashClone-8      5000000               180.4 ns/op            64 B/op          4 allocs/op
BenchmarkStrictSlashParse-8      5000000               573.5 ns/op           264 B/op          8 allocs/op
PASS
ok      github.com/gorilla/mux  3.788s

$ go test -benchmem -benchtime 5000000x -bench BenchmarkStrictSlash
goos: linux
goarch: amd64
pkg: github.com/gorilla/mux
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
BenchmarkStrictSlashClone-8      5000000               175.8 ns/op            64 B/op          4 allocs/op
BenchmarkStrictSlashParse-8      5000000               569.4 ns/op           264 B/op          8 allocs/op
PASS
ok      github.com/gorilla/mux  3.744s
```

```
func BenchmarkStrictSlashClone(b *testing.B) {
	req := newRequest(http.MethodGet, "http://localhost/x")
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = replaceURLPath(req.URL, req.URL.Path+"/")
	}
}

func BenchmarkStrictSlashParse(b *testing.B) {
	req := newRequest(http.MethodGet, "http://localhost/x")
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		u, _ := url.Parse(req.URL.String())
		u.Path += "/"
		_ = u.String()
	}
}
```

Signed-off-by: Jakob Ackermann <[email protected]>

mux.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"net/url"
 	"path"
 	"regexp"
 )
@@ -180,15 +181,7 @@ func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		}
 		// Clean path to canonical form and redirect.
 		if p := cleanPath(path); p != path {
-
-			// Added 3 lines (Philip Schlump) - It was dropping the query string and #whatever from query.
-			// This matches with fix in go 1.2 r.c. 4 for same problem.  Go Issue:
-			// http://code.google.com/p/go/issues/detail?id=5252
-			url := *req.URL
-			url.Path = p
-			p = url.String()
-
-			w.Header().Set("Location", p)
+			w.Header().Set("Location", replaceURLPath(req.URL, p))
 			w.WriteHeader(http.StatusMovedPermanently)
 			return
 		}
@@ -488,6 +481,14 @@ func cleanPath(p string) string {
 	return np
 }
 
+// replaceURLPath prints an url.URL with a different path.
+func replaceURLPath(u *url.URL, p string) string {
+	// Operate on a copy of the request url.
+	u2 := *u
+	u2.Path = p
+	return u2.String()
+}
+
 // uniqueVars returns an error if two slices contain duplicated strings.
 func uniqueVars(s1, s2 []string) error {
 	for _, v1 := range s1 {

regexp.go

@@ -355,13 +355,14 @@ func (v routeRegexpGroup) setMatch(req *http.Request, m *RouteMatch, r *Route) {
 			p1 := strings.HasSuffix(path, "/")
 			p2 := strings.HasSuffix(v.path.template, "/")
 			if p1 != p2 {
-				u, _ := url.Parse(req.URL.String())
+				p := req.URL.Path
 				if p1 {
-					u.Path = u.Path[:len(u.Path)-1]
+					p = p[:len(p)-1]
 				} else {
-					u.Path += "/"
+					p += "/"
 				}
-				m.Handler = http.RedirectHandler(u.String(), http.StatusMovedPermanently)
+				u := replaceURLPath(req.URL, p)
+				m.Handler = http.RedirectHandler(u, http.StatusMovedPermanently)
 			}
 		}
 	}