BUG: Index.str.partition gives ValueError while trying to compute index name (expand=False)
`Index.str.partition`'s behaviour changed in 3ab35b40274276e4f9b40918a7e260f1abb941c1 in a backwards incompatible way which doesn't seem intentional.
#### Code Sample, a copy-pastable example if possible

```
python3 -c 'import pandas; pandas.Index(["a,b", "c,d"], name="hello").str.partition(",")'
```

``` pytb
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/pwaller/.local/src/pandas/pandas/core/strings.py", line 1432, in partition
    return self._wrap_result(result, expand=expand)
  File "/home/pwaller/.local/src/pandas/pandas/core/strings.py", line 1348, in _wrap_result
    return MultiIndex.from_tuples(result, names=name)
  File "/home/pwaller/.local/src/pandas/pandas/indexes/multi.py", line 889, in from_tuples
    return MultiIndex.from_arrays(arrays, sortorder=sortorder, names=names)
  File "/home/pwaller/.local/src/pandas/pandas/indexes/multi.py", line 844, in from_arrays
    names=names, verify_integrity=False)
  File "/home/pwaller/.local/src/pandas/pandas/indexes/multi.py", line 92, in __new__
    result._set_names(names)
  File "/home/pwaller/.local/src/pandas/pandas/indexes/multi.py", line 446, in _set_names
    raise ValueError('Length of names must match number of levels in '
ValueError: Length of names must match number of levels in MultiIndex.
```
#### Expected Output

``` python
In [1]: import pandas; pandas.Index(["a,b", "c,d"], name="hello").str.partition(",")
Out[1]: 
MultiIndex(levels=[['a', 'c'], [','], ['b', 'd']],
           labels=[[0, 1], [0, 0], [0, 1]])
```
#### Versions

I bisected the problem to 3ab35b40274276e4f9b40918a7e260f1abb941c1 (cc @sinhrks). The problem is not present in 0.17.1 and is present in 0.18.0rc2. 

Note: when I use a `name="foo"`, it happens to work because it has the correct length, the index name ends up being `["f", "o", "o"]`, so it's possible this is tested somewhere and happens to work even though it's not correct for a large number of use cases.
