pandas.DataFrame.resample  behaves inconsistently when the rule is set to 24H/1D and 48H/2D
#### Code Sample, a copy-pastable example if possible

```python
>>> pd.__version__
'0.23.4'
>>> index = pd.date_range('11/11/2000 06:00:00', periods=50, freq='H')
>>> series = pd.Series(range(50), index=index)
>>> series.head(10)
2000-11-11 06:00:00    0
2000-11-11 07:00:00    1
2000-11-11 08:00:00    2
2000-11-11 09:00:00    3
2000-11-11 10:00:00    4
2000-11-11 11:00:00    5
2000-11-11 12:00:00    6
2000-11-11 13:00:00    7
2000-11-11 14:00:00    8
2000-11-11 15:00:00    9
Freq: H, dtype: int64
>>> series.resample('24H').count()
2000-11-11    18
2000-11-12    24
2000-11-13     8
Freq: 24H, dtype: int64
>>> series.resample('1D').count()
2000-11-11    18
2000-11-12    24
2000-11-13     8
Freq: D, dtype: int64
>>> series.resample('48H').count()
2000-11-11    42
2000-11-13     8
Freq: 48H, dtype: int64
>>> series.resample('2D').count()
2000-11-11 06:00:00    48
2000-11-13 06:00:00     2
dtype: int64

```
#### Problem description

As you can see, when I set rule to '24H' or '1D', the behavior of resample is consistent (in both cases it starts at 0 o'clock on the first day), but for '48H' and '2D' it's obviously not.

#### Expected Output
If the behavior consistency of ‘24H’ and ‘1D’ is correct, then the behaviors of '48H' and '2D' should also be consistent.
#### Output of ``pd.show_versions()``

<details>

commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.23.4
pytest: None
pip: 18.1
setuptools: 40.4.3
Cython: None
numpy: 1.15.2
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
