Revert "as.factor is optional for text columns in regressions"

This reverts commit b2acd8911f7b9a11b3f182ea528846c2748aeecd.
Revert: automaticall convert text into factor

It is not so simple to implement this, and it is reasonable not to
implement this. R's data.frame automatically converts text columns into
factors when it is created, while db.data.frame does not do that.