base R as function is weird

[Richard Cotton]

For example

as(1:5, "double")
#not the same as as.double(1:5)

as(1:5, "data.frame")
#fails!

Write as2 that uses as.someclass(x) when it exists and as(x, "someclass") otherwise.

Make coerce_to call it.

See also is2.

[Richard Cotton]

Fixed in development version by updating coerce_to.