websocket: use of crypto/rand package in test

The Read function of math/rand package doesn't exist on Go 1.5 or below.

Change-Id: If837bbd0a862726be93e07b021701004ba0f0fad
Reviewed-on: https://go-review.googlesource.com/30896
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

websocket/websocket_test.go

@@ -6,10 +6,10 @@ package websocket
 
 import (
 	"bytes"
+	"crypto/rand"
 	"fmt"
 	"io"
 	"log"
-	"math/rand"
 	"net"
 	"net/http"
 	"net/http/httptest"