Assigning multi-index DataFrame to Panel index throws exception
#### Code Sample, a copy-pastable example if possible

```
mi = pandas.MultiIndex.from_tuples([('c', 'd'), ('e', 'd'), ('f', 'g')])
df = DataFrame({'a': [1,2,3], 'b': [1,5,6]}, index=mi)
p = Panel(data={'one': df}).transpose(1,2,0)
p.loc[:, :, 'one'] = p.loc[:, :, 'one']
```

I get:

```
TypeError: Expected tuple, got str
```

In the original case, where indices are (str, timestamp) I get:

```
ValueError: Buffer dtype mismatch, expected 'Python object' but got 'long'
```

Apparently I'm doing something wrong, as this works:

```
p = Panel(data={'one': df})
p.loc['one', :, :] = p.loc['one', :, :]
```
#### Output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.1
setuptools: 20.7.0
Cython: 0.24
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.6
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext)
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
```
