Support mapping logical values into the closest supported type

[scw]

As reported by Andrew Gilbert, when writing out logical (boolean) values with arc.write(), NA values will instead be written out as -1, which as.logical() will evaluate as TRUE. Logicals in R have the following possible values:

Name	Possible values
True	TRUE, T, 1
False	FALSE, F, 0
Not Available	NA

While NA and NULL are actually different concepts, for the purposes of the bridge, they can be treated as equivalent for data exchange. It'd make sense to retain these either as:

• A string, with the values TRUE, FALSE and NULL. This is the best choice from an R perspective, as its tools can directly interpret these as logical values.
• A short, with the values 1, 0, and NULL. R won't automatically recognize these are logical values, but they can be cast to the correct type with as.logical.