Incorrect result with DateOffset.rollback for normalize=True
#### Code Sample, a copy-pastable example if possible

```python
from pandas.tseries.offsets import MonthBegin
MonthBegin(normalize=True).rollback('2020-4-1 12:34')
Out[2]: Timestamp('2020-03-01 00:00:00')
```

#### Problem description
It rollbacks to the beginning of the previous month instead of the current month. This happens only when normalize=True and the timestamp is within the first day of the month but strangely, it's correct for midnight:
```python
from pandas.tseries.offsets import MonthBegin
MonthBegin(normalize=True).rollback('2020-4-1 00:00')
Out[3]: Timestamp('2020-04-01 00:00:00')
```

#### Expected Output
```
Out[2]: Timestamp('2020-04-01 00:00:00')
```
In general, I would expect the following to hold for any timestamp `dt`:
```python
assert MonthBegin(normalize=True).rollback(dt) == MonthBegin().rollback(dt.normalize())
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 40.6.2
Cython           : 0.29.4
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.6.1
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.2.0
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : 0.2.1
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : 0.8.6
xarray           : 0.15.0
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.8
numba            : None

</details>
