Series to_string does not show index name

The standard `repr` for a `Series` shows the index name:

```
repr(pd.Series([1], index=pd.Index(["a"], name="idx")))
```

```
idx
a      1
dtype: int64
```

but `to_string` does not:

```
pd.Series([1], index=pd.Index(["a"], name="idx")).to_string(name=True)
```

```
a      1
```
