Fix `tiledb_array_schema()` enumeration mapping

[cgiachalis]

This PR fixes #853.

For complete testing please use the included unit tests.

library(tiledb)

dom <- tiledb_domain(c(tiledb_dim(
  name = "id",
  domain = c(NULL, NULL),
  tile = NULL,
  type = "ASCII")))

attrs <- c(
  
  tiledb_attr(
    name = "col1",
    type = "INT32",
    ncells = 1,
    nullable = FALSE),
  tiledb_attr(
    name = "enum1",
    type = "INT32",
    ncells = 1,
    nullable = FALSE,
    enumeration = TRUE
  ),
  tiledb_attr(
    name = "col2",
    type = "INT32",
    ncells = 1,
    nullable = FALSE
  ),
  tiledb_attr(
    name = "enum2",
    type = "INT32",
    ncells = 1,
    nullable = FALSE,
    enumeration = TRUE
  ),
  tiledb_attr(
    name = "enum3",
    type = "INT32",
    ncells = 1,
    nullable = FALSE,
    enumeration = TRUE
  )
)

# case 1 (ordered enums)
uri <- tempfile()

enums <- list(
  enum1 =  c("A", "B"),
  enum2 = c("yes", "no"),
  enum3 = c("aa")
)

sch <- tiledb_array_schema(domain = dom, attrs = attrs, sparse = TRUE, enumerations = enums)
tiledb_array_create(uri, sch)
arr <- tiledb_array(uri)

tiledb_array_has_enumeration(arr)

Before

#>  col1 enum1  col2 enum2 enum3 
#>  TRUE  TRUE  TRUE  TRUE  TRUE

After

#>  col1 enum1  col2 enum2 enum3 
#> FALSE  TRUE FALSE  TRUE  TRUE