DEPR: DataFrame.combine propagates nan values
The documentation for DataFrame.combine claims that the method "do[es] not propagate NaN values, so if for a (column, time) one frame is missing a value, it will default to the other frame’s value". However, this does not seem to correspond to the actual behaviour of DataFrame.combine.

Sample code:

``` python
>>> import pandas as pd
>>> from operator import add
>>> a = pd.DataFrame({
...        'a': pd.Series([1, 3], index=[0, 1]),
...        'b': pd.Series([2, 3], index=[1, 2]),
...    })
>>> b = pd.DataFrame({
...         'a': pd.Series([3, 5], index=[1, 2]),
...         'c': pd.Series([1, 2, 3], index=[2, 3, 4])
...     })
>>> a.combine(b, add)
    a   b   c
0 NaN NaN NaN
1   6 NaN NaN
2 NaN NaN NaN
3 NaN NaN NaN
4 NaN NaN NaN
```

In many cases (as in this one), it may be remedied by using the _fill_value_ parameter of DataFrame.combine. However, it might be a problem when there is no acceptable neutral element for the given combining function.

``` python
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Windows
OS-release: 8
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.16.2
nose: 1.3.7
Cython: 0.22
numpy: 1.9.2
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 3.1.0
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.6
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
```
