Single-level tuple index to multi-level index on pd.concat
```python
import pandas as pd
import numpy as np

s1 = pd.Series(np.random.randn(2), index=[('a', 'b'), ('x', 'y', 'z')])
s1.index.name = 'Idx'
s1.name = 's1'

s2 = pd.Series(np.random.randn(2), index=[('a', 'b'), ('j', 'k', 'l')])
s2.index.name = 'Idx'
s2.name = 's2'

# Result
pd.concat([s1, s2], axis=1, sort=False)
```
#### Problem description
Concatenating two series with a tuple as index, results in a multi-indexed dataframe

#### Expected Output
```
s1.to_frame().join(s2)
```
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.0.dev0+1028.gdb2066b7d
pytest: 3.8.1
pip: 18.1
setuptools: 40.2.0
Cython: 0.28.5
numpy: 1.15.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 5.0.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 2.2.3
openpyxl: 2.5.6
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.11
pymysql: 0.9.2
psycopg2: 2.7.5 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
