Unexpected behavior in DataFrame.shift(..., axis=1) with missing data
#### Code Sample
```python
import numpy as np
import pandas as pd

def check_shift(t):
    print('Original:', t, sep='\n')
    a = t.shift(-5, axis=1)
    print('Incorrect result:', a, sep='\n')
    b = t.transpose().shift(-5).transpose()
    print('Correct result:', b, sep='\n')
    return(a.equals(b))

# create dummy dataframe
df = pd.DataFrame(np.random.randint(0, 1, size=(2, 10)))
# add some missing values 
df.iloc[0, 7:10] = np.NaN
# below evaluates to false, but should be true
print('Test passed:', check_shift(df))
```
#### Problem description

The shift operation with `axis=1` produces unexpected results when the underlying dataframe contains missing values. 

The issue is that unexpected additional missing values get inserted with the shift operation.


#### Expected Output
`print(check_shift(df))` should return true
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
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: 32.2.0
Cython: None
numpy: 1.13.1
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: 0.4.0
matplotlib: 2.0.2
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.10
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
