statistical reducers output mismatch

The output for statistical reducers when used in the square brackets syntax does not match that of Frame methods.

```python
srcs = ["a", "bc", "def", None, -2.5, 3.7] / dt.obj64
df = dt.Frame(srcs)
RES1 = df.min()
RES2 = df[:, dt.min(f[:])] # What is the expected output here?
```

RES1 and RES2 should match but RES2 gives `TypeError: Unable to apply reduce function min() to a column of type obj64` instead. This mismatch is seen for `max`, `mean` and likely for other reducers too. As written in the comments in line 4, what is the expected output there?