Allow assignment into xpose_data class without conversion to uneval

[billdenney]

Allow assignment into the xpose_data class without conversion to an uneval class. (This is the change I intended to send in the last PR.)

[bguiastr]

Great @billdenney thanks a lot for the contribution!

[bguiastr]

I still getting issues with the assignment. Here is a reprex:

xpdb <- xpdb_ex_pk
class(xpdb)
# > "xpose_data" "uneval"   

xpdb$options$quiet <- TRUE
class(xpdb)
# > "uneval"

[billdenney]

When I test it in that branch, it worked for me. I'm about to push a test to that branch, and I think since this is already merged, it requires a new PR.