Fix PUB-1075: NaN imputation of categoricals depended on ASTOp "mode", which
erroneously returned -1 instead of Double.NaN, because it was looking for the
string "NA" in the domain, which typically doesn't happen (arguably, should
never happen).

This caused NaN imputation of categoricals with method = "mode" to fill in a
value of -1, which led to _domain[-1] type errors during algo runtime.
Merge branch 'master' of https://github.com/0xdata/h2o