BUG: bug in json serialization when frame has mixed types
closes https://github.com/pydata/pandas/issues/10289

on master:

``` python
>>> df
   1st  2nd  3rd  4th   5th
a   10    1  foo  0.1  0.01
b   20    2  bar  0.2  0.02
c   30    3  baz  0.3  0.03
d   40    4  qux  0.4  0.04

>>> read_json(df.to_json(orient='index'), orient='index', convert_axes=False)
   1st  2nd  3rd  4th   5th
a   40    4  qux  0.4  0.04
```

on branch:

``` python
>>> read_json(df.to_json(orient='index'), orient='index', convert_axes=False)
   1st  2nd  3rd  4th   5th
a   10    1  foo  0.1  0.01
b   20    2  bar  0.2  0.02
c   30    3  baz  0.3  0.03
d   40    4  qux  0.4  0.04
```
