Write Excel Round-Trippable Timedeltas Regression
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd
from io import BytesIO

pd.__version__ # 0.22 or 0.23.1

import openpyxl
openpyxl.__version__ # 2.5.4

import xlsxwriter
xlsxwriter.__version__ # 1.0.5

df = pd.DataFrame({'Times': pd.TimedeltaIndex(['00:01:00', '00:02:00', '00:03:00', np.nan])})
memfile = BytesIO()
df.to_excel(memfile, engine='openpyxl') # 'xlsxwriter' or 'openpyxl'
memfile.seek(0)
reread = pd.read_excel(memfile)
reread.iloc[0] == df.iloc[0]
# raises TypeError (reread has float type) when engine='xlsxwriter' or pd.__version__ == '0.23.1'
# if pd.__version__ == '0.22' and engine='openpyxl' evaluates to False
```

#### Problem description

Writing a Dataframe with timedeltas to excel using the openpyxl engine has changed behaviour from pandas version 0.22 to 0.23.1. Previously, the excel file produced could be read in as a Dataframe again with values matching the original Dataframe. If the xlsxwriter engine or pandas 0.23.1 is used, the timedeltas become floats in the re-read Dataframe.

| Engine \ Pandas Version | 0.22  | 0.23.1  |
| ------------------------ |:-----:| -------:|
| openpyxl | √ | X |
| xlsxwriter | X | X |

#### Expected Output

One the one hand, it's nice that pandas now has consistent behaviour between engines, however I think the desired behaviour would be for both engines to match the old openpyxl behaviour rather than the old xlsxwriter behaviour, as this would enable Dataframes written to excel to be used again when read back from excel.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.22.0 or 0.23.1
pytest: 3.6.1
pip: 10.0.1
setuptools: 39.1.0
Cython: None
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: 0.10.3
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.4
blosc: None
bottleneck: None
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.4
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.5
lxml: None
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.8
pymysql: 0.8.1
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
