combine_first throws ValueError: Cannot convert NA to integer
I do not understand why there is a need to convert NA to integer if the result does not have NAs. Perhaps the combine_first algo needs to do it under the hood?


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

```python
from pandas import DataFrame
DataFrame({'a': [0, 1, 3, 5]}).combine_first(DataFrame({'a': [1, 4]}))

Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/IPython/core/interactiveshell.py", line 3066, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-16-12b973b1b150>", line 1, in <module>
    pd.DataFrame({'a': [0, 1, 3, 5]}).combine_first(pd.DataFrame({'a': [1, 4]}))
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/frame.py", line 3787, in combine_first
    return self.combine(other, combiner, overwrite=False)
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/frame.py", line 3714, in combine
    otherSeries = otherSeries.astype(new_dtype)
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/generic.py", line 3054, in astype
    raise_on_error=raise_on_error, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/internals.py", line 3168, in astype
    return self.apply('astype', dtype=dtype, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/internals.py", line 3035, in apply
    applied = getattr(b, f)(**kwargs)
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/internals.py", line 462, in astype
    values=values, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/pandas/core/internals.py", line 505, in _astype
    values = _astype_nansafe(values.ravel(), dtype, copy=True)
  File "/usr/local/lib/python3.4/dist-packages/pandas/types/cast.py", line 531, in _astype_nansafe
    raise ValueError('Cannot convert NA to integer')
ValueError: Cannot convert NA to integer
```

#### Expected Output
```
   a
0  0
1  1
2  3
3  5
```

It does work when at least one item is a float:

```python
DataFrame({'a': [0.0, 1, 3, 5]}).combine_first(DataFrame({'a': [1, 4]}))

     a
0  0.0
1  1.0
2  3.0
3  5.0
```

I am aware that integer series cannot have NAs but there is no need to introduce NAs here. I do like it that the series is not upcasted to float silently though.


#### Output of ``pd.show_versions()``
<details>

INSTALLED VERSIONS
------------------

commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.19.0-66-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0
nose: None
pip: 1.5.4
setuptools: 3.3
Cython: 0.24.1
numpy: 1.11.2
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.4.6
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
httplib2: 0.9.2
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None


</details>
