Dataframe.to_excel treats decimal.Decimal as string instead of numeric type, the data in the Excel cell is formatted as a string, not a number

#### Code Sample

(float output is prepared for the sake of comparison to Decimal)

```python
# ----------------------------------
### imports

import pandas as pd
import decimal as dc

# ----------------------------------
### definitions

value_str = '2.445'

# ----------------------------------
### instantiations

#df_float = pd.DataFrame(data=[float(value_str)])
df_decimal = pd.DataFrame(data=[dc.Decimal(value_str)])

# ----------------------------------
### exports

# excel
#df_float.to_excel('float' + '.xlsx')
df_decimal.to_excel('decimal' + '.xlsx')

```
#### Observed Output

(settings : German locale, decimal sign is ',')

```
	0
0	2.445
```

result and Excel warning : number formatted as string

#### Expected Output

(settings : German locale, decimal sign is ',')

```
	0
0	2,445
```


#### Problem description

Since Decimal represents a numeric type, one would expect that it gets treated as such, just like floats.

The exported value in the Excel cell should be a number, not a string.

Presumably, the openpyxl Cell type should get set to 'n'.

Seemingly, openpyxl is able to handle decimals and knows a numeric Cell type 'n' but pandas deliberately sets string/non-numeric type as the openpyxl Cell type when a Decimal value is set.


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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 2012ServerR2
machine: AMD64
processor: Intel64 Family 6 Model 79 Stepping 0, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.2
pytest: None
pip: 10.0.1
setuptools: 39.1.0
Cython: None
numpy: 1.16.2
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: 2.6.2
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
None

</details>

