to_timedelta('1', unit='h') does not work.
#### Code Sample, a copy-pastable example if possible

```python
# Your code here

In [8]: pd.to_timedelta('1', unit='h')
Out[8]: Timedelta('0 days 00:00:00.000000')

```
#### Problem description

I would expect all the following to return Timedelta('0 days 01:00:00').  pd.to_timedelta(1, unit='h') does not.  I must first cast the string '1' to a int or float then to a timedelta.

In [6]: pd.to_timedelta(1.0, unit='h')
Out[6]: Timedelta('0 days 01:00:00')

In [7]: pd.to_timedelta(1, unit='h')
Out[7]: Timedelta('0 days 01:00:00')

In [8]: pd.to_timedelta('1', unit='h')
Out[8]: Timedelta('0 days 00:00:00.000000')

In [9]: pd.to_timedelta('1 H')
Out[9]: Timedelta('0 days 01:00:00')

#### Expected Output

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

<details>
# Paste the output here pd.show_versions() here

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.20.2
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
