[r] $set_metadata() does not update metadata if it already exists

[mojaveazure]

Describe the bug
$set_metadata() does not update array-level metadata if it already exists; instead, it runs to completion without any indication that the metadata was not updated

To Reproduce

> library(tiledbsoma)
> uri <- withr::local_tempdir()
> arr <- SOMASparseNDArrayCreate(uri, type = arrow::int32(), shape = c(20L, 10L))
> 
> (md <- list(
+   metadata_1 = bit64::as.integer64(0L),
+   metadata_2 = bit64::as.integer64(10L)
+ ))
$metadata_1
integer64
[1] 0

$metadata_2
integer64
[1] 10

> arr$set_metadata(md)
> 
> arr$reopen("READ")
> for (i in names(md)) {
+   print(arr$get_metadata(i))
+ }
integer64
[1] 0
integer64
[1] 10

> (md2 <- `[[<-`(md, "metadata_2", bit64::as.integer64(20L)))
$metadata_1
integer64
[1] 0

$metadata_2
integer64
[1] 20

> arr$reopen("WRITE")$set_metadata(md)
> arr$reopen("READ")
> for (i in names(md)) {
+   print(arr$get_metadata(i))
+ }
integer64
[1] 0
integer64
[1] 10

Versions (please complete the following information):

> tiledbsoma::show_package_versions()
tiledbsoma:    1.9.99.3
tiledb-r:      0.25.0
tiledb core:   2.21.1
libtiledbsoma: 2.21.1
R:             R version 4.2.3 (2023-03-15)
OS:            Ubuntu 22.04.4 LTS