Setting 2D slice of a Panel
#### Issue

Assigning to a Panel with two vector indexers and one scalar indexer produces unexpected behaviour: the vector indexers are zipped together, rather than the Cartesian product.

#### A small, complete example of the issue

```python
p=pd.Panel(0,[0,1],[0,1],[0,1])
p.loc[[0,1],[0,1],[0]]=21
p.loc[[0,1],[0,1],0]=42
p.loc[[0,1],[0,1],0]
```

#### Actual Output
```
    0     1
0  42   21
1  21   42
```

#### Expected Output
```
     0   1
0  42  42
1  42  42
```

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: 06b35db1d88e75a9e3f183b58678589cf7715381
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.1.3-101.fc21.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.19.0+69.g06b35db
nose: 1.3.7
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.3.1
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None
</details>
