Styling of column and row headers not reflected when saving to excel.  
Following is the code I used to style the headers of both rows and columns and then saving the stylized data frame to an excel file.

```python
df = pd.DataFrame([1000000.0, 1000000.0], index=pd.bdate_range('2017-08-01', '2017-08-02'))

df.index = df.index.strftime("%Y-%m-%d")
styler = df.style
styler.set_table_styles(
# select the table header with th and set it right align
    [dict(selector="th", props=[("text-align", "right"),('color','green')])]   
)
styler.to_excel('styled.xlsx', engine='openpyxl')

```
#### Problem description

I am trying to stylize the headers of a data frame and then save that data frame to an excel file, but I am unable to see the stylized effects inside the excel file. 

#### Output of Jupyter Notebook 
![image](https://user-images.githubusercontent.com/9162599/52338222-cc3ead80-2a2f-11e9-92da-e051988693ca.png)


#### Output in excel file 
![image](https://user-images.githubusercontent.com/9162599/52338330-1e7fce80-2a30-11e9-9fbb-162bddd82c4a.png)

#### Expected Output
![image](https://user-images.githubusercontent.com/9162599/52339032-fe510f00-2a31-11e9-8826-3a3d5df214f7.png)


Pandas Version: '0.24.1'


