BUG: repr of Series with regular (freq) datetime index after reading with msgpack
From the io docs:

```
In [18]: s = pd.Series(np.random.rand(5), index=pd.date_range('20130101', periods=5))                                                                                           

In [19]: s                                                                                                                                                                      
Out[19]: 
2013-01-01    0.130706
2013-01-02    0.232104
2013-01-03    0.506547
2013-01-04    0.155568
2013-01-05    0.873604
Freq: D, dtype: float64

In [20]: s.to_msgpack('test.msg')                                                                                                                                               

In [22]: s2 = pd.read_msgpack('test.msg')                                                                                                                                       

In [23]: s2                                                                                                                                                                     
Out[23]: ---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
~/miniconda3/envs/dev/lib/python3.5/site-packages/IPython/core/formatters.py in __call__(self, obj)
    700                 type_pprinters=self.type_printers,
    701                 deferred_pprinters=self.deferred_printers)
--> 702             printer.pretty(obj)
    703             printer.flush()
    704             return stream.getvalue()

~/miniconda3/envs/dev/lib/python3.5/site-packages/IPython/lib/pretty.py in pretty(self, obj)
    400                         if cls is not object \
    401                                 and callable(cls.__dict__.get('__repr__')):
--> 402                             return _repr_pprint(obj, self, cycle)
    403 
    404             return _default_pprint(obj, self, cycle)

~/miniconda3/envs/dev/lib/python3.5/site-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
    695     """A pprint that just redirects to the normal repr function."""
    696     # Find newlines and replace them with p.break_()
--> 697     output = repr(obj)
    698     for idx,output_line in enumerate(output.splitlines()):
    699         if idx:

~/scipy/pandas/pandas/core/base.py in __repr__(self)
     75         Yields Bytestring in Py2, Unicode String in py3.
     76         """
---> 77         return str(self)
     78 
     79 

~/scipy/pandas/pandas/core/base.py in __str__(self)
     54 
     55         if compat.PY3:
---> 56             return self.__unicode__()
     57         return self.__bytes__()
     58 

~/scipy/pandas/pandas/core/series.py in __unicode__(self)
   1272 
   1273         self.to_string(buf=buf, name=self.name, dtype=self.dtype,
-> 1274                        max_rows=max_rows, length=show_dimensions)
   1275         result = buf.getvalue()
   1276 

~/scipy/pandas/pandas/core/series.py in to_string(self, buf, na_rep, float_format, header, index, length, dtype, name, max_rows)
   1316                                         float_format=float_format,
   1317                                         max_rows=max_rows)
-> 1318         result = formatter.to_string()
   1319 
   1320         # catch contract violations

~/scipy/pandas/pandas/io/formats/format.py in to_string(self)
    258     def to_string(self):
    259         series = self.tr_series
--> 260         footer = self._get_footer()
    261 
    262         if len(series) == 0:

~/scipy/pandas/pandas/io/formats/format.py in _get_footer(self)
    205 
    206         if getattr(self.series.index, 'freq', None) is not None:
--> 207             footer += 'Freq: {freq}'.format(freq=self.series.index.freqstr)
    208 
    209         if self.name is not False and name is not None:

~/scipy/pandas/pandas/core/arrays/datetimelike.py in freqstr(self)
    502         if self.freq is None:
    503             return None
--> 504         return self.freq.freqstr
    505 
    506     @property  # NB: override with cache_readonly in immutable subclasses

AttributeError: 'str' object has no attribute 'freqstr'
```