Regression in 0.24: to_timedelta handling of float values
#### Code Sample, a copy-pastable example if possible

Creating an TimedeltaIndex with 1 microsecond steps:

```python
import pandas as pd
import scipy as sp
t = sp.arange(0,1,1e-6)
index = pd.to_timedelta(t, unit='s')
```
#### Problem description

Output in 0.24:

![image](https://user-images.githubusercontent.com/20566367/52139038-40074180-264f-11e9-8513-f2bdbe7acaf2.png)

We have repeated index values (e.g. 99992 in the image) and missing ones (e.g. 99994 in the image).

This looks like some sort of rounding issue.

#### Expected Output

Output in 0.23.4:

![image](https://user-images.githubusercontent.com/20566367/52138931-eb63c680-264e-11e9-82b0-1719ad98aa0e.png)

In this  version everything is okay.

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

<details>

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.0
pytest: 4.1.0
pip: 18.1
setuptools: 39.0.1
Cython: 0.29.2
numpy: 1.15.4
scipy: 1.1.0
pyarrow: 0.11.1
xarray: 0.11.2
IPython: 7.2.0
sphinx: 1.8.3
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: 1.7.0
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: 3.0.2
openpyxl: 2.5.12
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml.etree: 4.3.0
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.2.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: 0.2.0
fastparquet: 0.2.1
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
