Series.str.cat not handling NA when using other

```python
Series(['a', 'b', 'c']).str.cat(['A', np.NaN, 'C'], sep=',')

0    a,A
1    NaN
2    c,C
dtype: object

```
#### Problem description

I was expecting NaN to be skipped and just get "b" in the second line. 

When using na_rep it does something like this.

```python
Series(['a', 'b', 'c']).str.cat(['A', np.NaN, 'C'], sep=',', na_rep='X')

0    a,A
1    b,X
2    c,C
dtype: object
```

I feel this behavior is inconsistent. 

I can see that if you are expecting to get output that could be used like a record/csv current behavior would help. Perhaps this could work as a parameter.

#### Expected Output

```python
0    a,A
1    b
2    c,C
dtype: object
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-59-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 8.1.2
setuptools: 28.2.0
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: 1.3.1
patsy: None
dateutil: 2.4.2
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.1
xlsxwriter: 0.9.6
lxml: None
bs4: None
html5lib: 0.999999999
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
boto: None
pandas_datareader: None

</details>
