BUG: Series datatypes not preserved in DataFrame constructor
There is an inconsistency apparently in the way dtypes are parsed when DataFrames are created from Series:

```
>>> import pandas as pd
>>> df = pd.DataFrame({'f':pd.Series([1.5,], dtype='float32'), 'i':pd.Series([1,], dtype='int32')})
>>> df.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 1 entries, 0 to 0
Data columns (total 2 columns):
f    1 non-null float32
i    1 non-null int32
dtypes: float32(1), int32(1)
memory usage: 16.0 bytes

>>> df2 = pd.DataFrame([pd.Series([1.5,], dtype='float32'), pd.Series([1,], dtype='int32')], columns=['f','i'])
>>> df2.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 2 columns):
f    0 non-null float64
i    0 non-null float64
dtypes: float64(2)
memory usage: 48.0 bytes

```
