webdav: remove Go 1.6 support, use std context

Fixes golang/go#21364

Change-Id: Ibfc6f5001d7038e4efd1f3fe8fc6d3fdded85551
Reviewed-on: https://go-review.googlesource.com/c/148438
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/webdav/file.go b/webdav/file.go
index 748118d..9f53778 100644
--- a/webdav/file.go
+++ b/webdav/file.go
@@ -5,6 +5,7 @@
 package webdav
 
 import (
+	"context"
 	"encoding/xml"
 	"io"
 	"net/http"
@@ -14,8 +15,6 @@
 	"strings"
 	"sync"
 	"time"
-
-	"golang.org/x/net/context"
 )
 
 // slashClean is equivalent to but slightly more efficient than