context: Use crypto/rand in case method used for crypto purposes

caddyhttp/httpserver/context.go

@@ -2,9 +2,9 @@ package httpserver
 
 import (
 	"bytes"
+	"crypto/rand"
 	"fmt"
 	"io/ioutil"
-	"math/rand"
 	"net"
 	"net/http"
 	"net/url"