BUG: series.truncate doesn't return the correct value in pandas 1.1.0
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#### Code Sample, a copy-pastable example

```python
def test_truncate():
    series = pd.Series([0.1], index=pd.DatetimeIndex(['2020-08-04']))
    before = pd.Timestamp('2020-08-02')
    after = pd.Timestamp('2020-08-04')

    assert series.truncate(before=before, after=after).equals(series)

```

#### Problem description

The above test passes in pandas 1.0.5 while fails in pandas 1.1.0. 

After diving into the code I found that *truncate* in pandas/core/generic.py changed as follows:
```
if ax.is_monotonic_decreasing:
     before, after = after, before
```
the above check was added to the method. ```ax.is_monotonic_decreasing``` returns true for 1-element series, (```ax.is_monotonic_increasing``` also returns true). When *before*, and *after* are given in the correct, increasing order (like in the test), the following check reverses them and the result of  truncate will be an empty series, instead of returning the input series which is the expected behaviour.