0.17 regression: convert_objects coercion with multiple dtypes
I upgraded from 0.16.2 to 0.17/master and it broke working code.

0.16.2:

``` python
In [2]: import pandas as pd
   ...: from StringIO import StringIO
   ...: x="""foo,bar
   ...: 2015-09-14,True
   ...: 2015-09-15,
   ...: """
   ...: df=pd.read_csv(StringIO(x),sep=',').convert_objects('coerce')
   ...: df.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 2 columns):
foo    2 non-null datetime64[ns]
bar    1 non-null object
dtypes: datetime64[ns](1), object(1)
memory usage: 48.0+ bytes

```

0.17/master:

``` python
import pandas as pd
from StringIO import StringIO
x="""foo,bar
2015-09-14,True
2015-09-15,
"""
df=pd.read_csv(StringIO(x),sep=',').convert_objects('coerce')
df.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 2 columns):
foo    2 non-null datetime64[ns]
bar    0 non-null datetime64[ns]
dtypes: datetime64[ns](2)
memory usage: 48.0 bytes
/home/ambk/work/pandas/pandas/core/generic.py:2584: FutureWarning: The use of 'coerce' as an input is deprecated. Instead set coerce=True.
  FutureWarning)
```

booleans get cast to datetimes now?! Usually deprecation means "avoid in new code" and not that your working code will break, otherwise it wouldn't be a deprecation but a breaking change. So that's not good, but ok, let's follow the helpful hint:

``` python
In [3]: import pandas as pd
   ...: from StringIO import StringIO
   ...: x="""foo,bar
   ...: 2015-09-14,True
   ...: 2015-09-15,
   ...: """
   ...: df=pd.read_csv(StringIO(x),sep=',').convert_objects(coerce=True)
   ...: df.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 2 columns):
foo    2 non-null object
bar    1 non-null float64
dtypes: float64(1), object(1)
memory usage: 48.0+ bytes
```

**wut?**, booleans become floats and almost-datetimes aren't converted at all anymore?
