DataFrame.agg: strange behavior
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df=pd.DataFrame([1, 2], columns=['a'])
ser=pd.Series([1, 3], name='a')
print(df)
print(df.agg({'a': lambda x: x.cumsum()}))
print(df.agg({'a': lambda x: ser}))
#################################################output
   a
0  1
1  2
   a
0  1
1  3
   a
   0  1
0  1  3
1  1  3

```
#### Problem description

Functions (lambda ...) which return the same Series, give different results. It seems that  somehow second agg was called with axis=1.

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

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.5-1-ARCH
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.2.2
pip: 9.0.1
setuptools: 36.5.0
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.3.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.5
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 4.1.1
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.6
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
None

</details>
