Does fillna work with boolean index?
#### Code

```python
>>> df
          A         B         C
0  1.739232  1.369457 -0.546500
1  0.340898       NaN  0.518132
2       NaN       NaN       NaN
3  1.868215  0.803584 -0.333043
>>> df.loc[~df.isnull().all(axis=1),'B'].fillna(99.9,inplace=True)
>>> df
          A         B         C
0  1.739232  1.369457 -0.546500
1  0.340898       NaN  0.518132
2       NaN       NaN       NaN
3  1.868215  0.803584 -0.333043
```
#### Problem description

I am trying to fill some null values in a column but not all using boolean indexing. But `fillna` doesn't seem to work in that case. What's the correct way to achieve it?

#### Expected Output
```python
>>> df
          A         B         C
0  1.739232  1.369457 -0.546500
1  0.340898      99.9  0.518132
2       NaN       NaN       NaN
3  1.868215  0.803584 -0.333043
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 16.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.12.1
scipy: 0.19.1
xarray: 0.9.6
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
