Timestamp.floor() rounds up when a few nanoseconds before a round time
#### Code Sample, a copy-pastable example if possible

```python
>>> pd.Timestamp('2001-02-03 04:05:06.99999999').floor('1s')
Timestamp('2001-02-03 04:05:07')

>>> pd.Timestamp('2004-05-06 07:08:59.9999999').floor('1min')
Timestamp('2004-05-06 07:09:00')
```
#### Problem description

For times very close to but below a round second, `pd.Timestamp.floor()` will round up instead of down. The threshold for where this happens varies from second to second but seems to be between 50 and 200 nanoseconds (i.e. `.9999998` will `floor()` correctly, `.99999995` will `floor()` to the next second upwards).

#### Expected Output
```
Timestamp('2001-02-03 04:05:06')
Timestamp('2004-05-06 07:08:00')
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-42-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.4
pytest: 4.0.1
pip: 18.1
setuptools: 40.6.2
Cython: 0.29
numpy: 1.15.4
scipy: 1.0.0
pyarrow: None
xarray: 0.11.0
IPython: 7.2.0
sphinx: 1.8.2
patsy: 0.5.0
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.5.11
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.14
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
