DataFrame slicing TimedeltaIndex with str
#### Code Sample, a copy-pastable example if possible

```python
cols = ['15:00:00', '15:01:00', '15:02:00', '15:02:30']
cols_td = pd.to_timedelta(cols)

df = pd.DataFrame(0, index=range(4), columns=cols_td)

vw1 = df.loc[:, :cols[-2]]
vw2 = df.loc[:, :cols_td[-2]]
print(vw1)
print(vw2)

print('Statement below must be equal')
print(vw1.equals(vw2))
```
#### Problem description

vw1 is wrong. vw2 is correct
Slicing TimdeltaIndex DataFrame with string doesn't seem to work. It doesn't take into account seconds 

#### Expected Output
Both views must be equal

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.3
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
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: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
