`DataFrame.resample()` breaks when rule parameter is of type `DateOffset`
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

# Some exemplary data
df = pd.DataFrame({'Timestamp': pd.date_range('2020-01', '2020-03', periods=5),
                   'Amount': [5, 10, 15, 20, 25]}, columns=['Timestamp', 'Amount'])

working = df.resample('MS', on='Timestamp').sum() # This one works

failing = df.resample(pd.DateOffset(months=1), on='Timestamp').sum() # This one fails
```
#### Problem description

According to the documentation, the `rule=` parameter of `DataFrame.resample()` can be of type `DateOffset`, `Timedelta` or `str`. Apparently, it actually expects some of the `DateOffset` subclasses (like `MonthEnd`, `BusinessDay`, etc.), because `rule_code` attribute is not defined in the parent class. Hence, the error:
```
  File "~/.conda/envs/py37-simple/lib/python3.7/site-packages/pandas/core/generic.py", line 8114, in resample
    level=level,
  File "~/.conda/envs/py37-simple/lib/python3.7/site-packages/pandas/core/resample.py", line 1269, in resample
    tg = TimeGrouper(**kwds)
  File "~/.conda/envs/py37-simple/lib/python3.7/site-packages/pandas/core/resample.py", line 1341, in __init__
    rule = freq.rule_code
  File "~/.conda/envs/py37-simple/lib/python3.7/site-packages/pandas/tseries/offsets.py", line 460, in rule_code
    return self._prefix
  File "~/.conda/envs/py37-simple/lib/python3.7/site-packages/pandas/tseries/offsets.py", line 456, in _prefix
    raise NotImplementedError("Prefix not defined")
NotImplementedError: Prefix not defined
```

#### Expected Output
The `resample()` method is expected to work with plain `DateOffset` instances passed for `rule=` attribute.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8
pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 44.0.0.post20200106
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
