crypt: fix compress wrapping crypt giving upload errors

Before this fix a chain compress -> crypt -> s3 was giving errors

    BadDigest: The Content-MD5 you specified did not match what we received.

This was because the crypt backend was encrypting the underlying local
object to calculate the hash rather than the contents of the metadata
stream.

It did this because the crypt backend incorrectly identified the
object as a local object.

This fixes the problem by making sure the crypt backend does not
unwrap anything but fs.OverrideRemote objects.

See: https://forum.rclone.org/t/not-encrypting-or-compressing-before-upload/32261/10

backend/crypt/crypt.go

@@ -1047,10 +1047,11 @@ func (o *ObjectInfo) Hash(ctx context.Context, hash hash.Type) (string, error) {
 	// Get the underlying object if there is one
 	if srcObj, ok = o.ObjectInfo.(fs.Object); ok {
 		// Prefer direct interface assertion
-	} else if do, ok := o.ObjectInfo.(fs.ObjectUnWrapper); ok {
-		// Otherwise likely is an operations.OverrideRemote
+	} else if do, ok := o.ObjectInfo.(*fs.OverrideRemote); ok {
+		// Unwrap if it is an operations.OverrideRemote
 		srcObj = do.UnWrap()
 	} else {
+		// Otherwise don't unwrap any further
 		return "", nil
 	}
 	// if this is wrapping a local object then we work out the hash

backend/crypt/crypt_internal_test.go

@@ -17,19 +17,6 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-type testWrapper struct {
-	fs.ObjectInfo
-}
-
-// UnWrap returns the Object that this Object is wrapping or nil if it
-// isn't wrapping anything
-func (o testWrapper) UnWrap() fs.Object {
-	if o, ok := o.ObjectInfo.(fs.Object); ok {
-		return o
-	}
-	return nil
-}
-
 // Create a temporary local fs to upload things from
 
 func makeTempLocalFs(t *testing.T) (localFs fs.Fs, cleanup func()) {
@@ -83,7 +70,7 @@ func testObjectInfo(t *testing.T, f *Fs, wrap bool) {
 	var oi fs.ObjectInfo = obj
 	if wrap {
 		// wrap the object in an fs.ObjectUnwrapper if required
-		oi = testWrapper{oi}
+		oi = fs.NewOverrideRemote(oi, "new_remote")
 	}
 
 	// wrap the object in a crypt for upload using the nonce we