Default implementation for dbDataType()

[krlmlr]

• Doesn't work with arguments of unknown class ("AsIs", "unknown1"), as required by DBItest
  • I don't remember why I added this requirement, but it may be better to err on the safe side here
• Need implementation for lists (Lists of raw vectors should map to blobs by default #49)
• Need implementation for hms/difftime (Think about return types for date, time and timestamp #42)
• Should return objects of class SQL (https://github.com/rstats-db/RPostgres/pull/45#issuecomment-200960432)
• For data frames, a named list should be returned

[imanuelcostigan]

Default implementation fails DBItest for AsIs Date and DateTime. Not sure whether this is an issue for

dbDataType(ANSI(), I(Sys.Date()))
#> [1] "DOUBLE"

Maybe something like this in as_is_data_type()?

dd <- I(Sys.Date())
class(dd) <- class(dd)[-1]