Unexpected result when setting a row by a dict
#### Code Sample, a copy-pastable example if possible

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


persons = [
    {
        'name': ''.join([
            np.random.choice([chr(x) for x in range(97, 97 + 26)])
            for i in range(5)
        ]),
        'age': np.random.randint(0, 100),
        'sex': np.random.choice(['male', 'female']),
        'job': np.random.choice(['staff', 'cook', 'student']),
        'birthday': np.random.choice(pd.date_range('1990-01-01', '2010-01-01')),
        'hobby': np.random.choice(['cs', 'war3', 'dota'])
    }
    for i in range(10)
]

df = pd.DataFrame(persons)
df.set_index('birthday', inplace=True)
print(df)
df.iloc[0] = {
    'name': 'john',
    'age': int(10),
    'sex': 'male',
    'hobby': 'nohobby',
    'job': 'haha'
}
print(df)
```
#### Problem description

```
             age hobby      job   name     sex
birthday
2007-12-31  name   age      sex  hobby     job
2004-07-31    20  dota  student  uwxhn  female
2001-10-22    34  war3     cook  udknv  female
2002-10-13    91  dota     cook  bofcv  female
1992-05-25    54  war3     cook  tcqew    male
2009-09-02    95  war3    staff  jcolr  female
1998-12-15    61  war3  student  dibkw  female
2004-07-03     4  war3  student  mntqh    male
2000-06-08    88  war3    staff  jknxm  female
2006-10-19    82    cs  student  asrpz    male

```

Have no idea why the keys are set to the rows.

#### Expected Output
```
             age hobby      job   name     sex
birthday  
2007-12-31    10 nohobby  haha john male
2004-07-31    20  dota  student  uwxhn  female
2001-10-22    34  war3     cook  udknv  female
2002-10-13    91  dota     cook  bofcv  female
1992-05-25    54  war3     cook  tcqew    male
2009-09-02    95  war3    staff  jcolr  female
1998-12-15    61  war3  student  dibkw  female
2004-07-03     4  war3  student  mntqh    male
2000-06-08    88  war3    staff  jknxm  female
2006-10-19    82    cs  student  asrpz    male
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 16.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 35.0.2
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: 0.9.5
IPython: 6.0.0
sphinx: 1.5.5
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
