Fix dbDataType() for AsIs object

[yutannihilation]

Current implementation of dbiDataType() for AsIs fails to restore the original class attributes. (I found this when I was trying to cheat DBItest with ANSI()...)

library(DBI)
library(DBItest)

dbObj <- structure(list(), class = "DBIDriver")
testthat::expect_equal(
  dbDataType(dbObj, I(structure(17455, class = "Date"))),
  dbDataType(dbObj, structure(17455, class = "Date"))
)
#> Error: dbDataType(dbObj, I(structure(17455, class = "Date"))) not equal to dbDataType(dbObj, structure(17455, class = "Date")).
#> 1/1 mismatches
#> x[1]: "DOUBLE"
#> y[1]: "DATE"

unclass() is OK for implicit classes (e.g. numeric and integer), but Date needs to be set as class attribute explicitly. I believe oldClass(x) <- oldClass(x)[-1] is the reverse of what I() does.

body(selectMethod("dbiDataType", "AsIs"))
#> {
#>     .local <- function (x) 
#>     {
#>         dbiDataType(unclass(x))
#>     }
#>     .local(x, ...)
#> }

body(I)
#> {
#>     structure(x, class = unique(c("AsIs", oldClass(x))))
#> }

[codecov-io]

Codecov Report

Merging #198 into master will increase coverage by 0.54%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   42.97%   43.52%   +0.54%     
==========================================
  Files          16       16              
  Lines         470      471       +1     
==========================================
+ Hits          202      205       +3     
+ Misses        268      266       -2

Impacted Files	Coverage Δ
R/data-types.R	41.17% <100%> (+16.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f8c97f2...c47c98a. Read the comment docs.

[yutannihilation]

I expected the first commit should fail, but maybe I pushed the second commit too early...

https://travis-ci.org/rstats-db/DBI/jobs/288387072

[krlmlr]

Thanks!