serve webdav: make Content-Type without reading the file and add --etag-hash

Before this change x/net/webdav would open each file to find out its
Content-Type.

Now we override the FileInfo and provide that directly from rclone.

An --etag-hash has also been implemented to override the ETag with the
hash passed in.

Fixes #2273

cmd/serve/webdav/webdav_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/ncw/rclone/cmd/serve/httplib"
 	"github.com/ncw/rclone/fstest"
 	"github.com/stretchr/testify/assert"
+	"golang.org/x/net/webdav"
 )
 
 const (
@@ -23,6 +24,13 @@ const (
 	testURL         = "http://" + testBindAddress + "/"
 )
 
+// check interfaces
+var (
+	_ os.FileInfo         = FileInfo{nil}
+	_ webdav.ETager       = FileInfo{nil}
+	_ webdav.ContentTyper = FileInfo{nil}
+)
+
 // TestWebDav runs the webdav server then runs the unit tests for the
 // webdav remote against it.
 func TestWebDav(t *testing.T) {