Support set the key of index from "index" to df.index.name in df.to_dict() when orient="split"
#### Code Sample, a copy-pastable example if possible
When I use to_dict() to convert the dataframe to dict in "split" mode. I just found that the key of index can not be changed. Is that possibile that to add a support: change the key of index from "index" to df.index.name or any way that I can change the key of index?
```python
df.to_dict(orient="split")
#Now what looks like is 
#result is
{'columns': [something,],
 'data': [[something],
  [something],
  [something],
 'index': [something]}


#what I want is
df.index.name = "name" #or some other ways that I can change the "index"
 
{'columns': [something,],
 'data': [[something],
  [something],
  [something],
 'name': [something]}
```
