Series not honoring class __repr__ or __str__
#### Code Sample, a copy-pastable example if possible

```
class foo(dict):
    def __init__(self, iterable=None, **kwargs):
        if iterable is None:
            iterable = ()
        super(foo, self).__init__(iterable)
        self.update(kwargs)
    def __repr__(self):
        return ",".join(self.keys())
    def __str__(self):
        return ",".join(self.keys())

f = foo({'alpha' : 'b',
    'beta' : 'c'})

import pandas as pd
pd.DataFrame(data=[['A', 1, f]], columns=['D', 'F', 'G'])

```
#### Problem description

For a given series with a custom object, I want to control the content when displayed via print or displaying on ipython notebooks.  The object ```foo``` is a simple class that have the ```__str__``` and ```__repr__``` overwritten, but still displays the object's dictionary content, not the view I want to show the end users.  How do I control that?



#### Expected Output

```alpha,beta```

what I get is:

```{'alpha': 'b', 'beta': 'c'}```

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.21.1
pytest: 3.3.1
pip: 9.0.1
setuptools: 38.2.4
Cython: None
numpy: 1.11.2
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 5.3.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 1.5.3
openpyxl: None
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
