vfs: fix applying modtime for an open Write Handle

The symptom of this was that the time set when the file was open was
lost.  This was causing one of the mount tests to fail too.

vfs/read_write_test.go

@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 	"testing"
+	"time"
 
 	"github.com/ncw/rclone/fs"
 	"github.com/ncw/rclone/fstest"
@@ -562,3 +563,27 @@ func TestRWFileHandleOpenTests(t *testing.T) {
 		testRWFileHandleOpenTest(t, vfs, &test)
 	}
 }
+
+// tests mod time on open files
+func TestRWFileModTimeWithOpenWriters(t *testing.T) {
+	r := fstest.NewRun(t)
+	defer r.Finalise()
+	vfs, fh := rwHandleCreateWriteOnly(t, r)
+
+	mtime := time.Date(2012, 11, 18, 17, 32, 31, 0, time.UTC)
+
+	_, err := fh.Write([]byte{104, 105})
+	require.NoError(t, err)
+
+	err = fh.Node().SetModTime(mtime)
+	require.NoError(t, err)
+
+	err = fh.Close()
+	require.NoError(t, err)
+
+	info, err := vfs.Stat("file1")
+	require.NoError(t, err)
+
+	// avoid errors because of timezone differences
+	assert.Equal(t, info.ModTime().Unix(), mtime.Unix())
+}