Partial string indexing returns ndarray rather than Series.
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np

print(pd.__version__)

df_pass = pd.DataFrame(index=range(1,1000), columns=['A', 'B', 'C'])
df_pass.loc[:, :] = np.random.uniform(-100, 100, size=(len(df_pass.index), len(df_pass.columns)))
print(df_pass.loc[range(1,500), 'A'].sum(skipna=False)) # everything is fine here

df_fail = pd.DataFrame(index=pd.date_range('01-01-2005', '12-01-2006'), columns=['A', 'B', 'C'])
df_fail .loc[:, :] = np.random.uniform(-100, 100, size=(len(df_fail .index), len(df_fail .columns)))
print(df_fail .loc['2005', 'A'].sum(skipna=False)) # Here the type-error appears
```

Output:
```
0.25.0
-847.9947710494175
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-20-26bdb9fa00f7> in <module>()
     10 df_fail = pd.DataFrame(index=pd.date_range('01-01-2005', '12-01-2006'), columns=['A', 'B', 'C'])
     11 df_fail .loc[:, :] = np.random.uniform(-100, 100, size=(len(df_fail .index), len(df_fail .columns)))
---> 12 print(df_fail .loc['2005', 'A'].sum(skipna=False)) # Here the type-error appears
     13
```

#### Problem description

Before updating from 0.24.0 to 0.25.0 everything worked fine. I can also not see, that there was an API change here. I would expect, that the second sum, works without issues.

#### Expected Output

Output (something like):
```
0.25.0
-847.9947710494175
-451.5691327012012
```

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

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.0
numpy            : 1.14.3
pytz             : 2018.4
dateutil         : 2.7.3
pip              : 19.1.1
setuptools       : 39.1.0
Cython           : 0.28.2
pytest           : 3.10.0
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.0.5
lxml.etree       : 4.2.5
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.7.6.1 (dt dec pq3 ext lo64)
jinja2           : 2.10
IPython          : 6.4.0
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.2.5
matplotlib       : 2.2.2
numexpr          : None
odfpy            : None
openpyxl         : 2.6.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : 1.2.12
tables           : None
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : 1.0.5
```
