Rounding error in Timestamp.floor()
#### Code Sample, a copy-pastable example if possible

```python
>>> pd.Timestamp('2117-01-01 00:00:45').floor('15s')
Timestamp('2117-01-01 00:00:30')

>>> pd.Timestamp('1823-01-01 00:00:01').floor('1s')
Timestamp('1823-01-01 00:00:00')
```
#### Problem description

For some timestamps more than a hundred years or so in the past or future, `Timestamp.floor()` rounds an already-round timestamp down to the next round value. I came up against the problem because I was assuming that `Timestamp.floor()` is idempotent.

I've looked at #14572 and #14440, these have both been closed as fixed but this bug is still present.

#### Expected Output
```python
>>> pd.Timestamp('2117-01-01 00:00:45').floor('15s')
Timestamp('2117-01-01 00:00:45')

>>> pd.Timestamp('1823-01-01 00:00:01').floor('1s')
Timestamp('1823-01-01 00:00:01')
```

#### Output of ``pd.show_versions()``
<details>
```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-42-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.22.0
pytest: 3.3.0
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: 0.10.0
IPython: 6.2.1
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```

</details>
