reset_index loses type information on time indices with time zone information
```
import pandas as pd

ts1 = pd.Timestamp('2015-07-01 10:00:00')
ts2 = pd.Timestamp('2015-07-01 11:00:00')

df1 = pd.DataFrame([[0.1],[0.2]], index=[ts1, ts2], columns=['a'])
print(df1.index.dtype)
df1 = df1.reset_index(drop=False)
print(df1['index'].dtype)

df2 = pd.DataFrame([[0.1],[0.2]], index=[ts1, ts2], columns=['a'])
df2.index = df2.index.tz_localize('Europe/London')
print(df2.index.dtype)
df2 = df2.reset_index(drop=False)
print(df2['index'].dtype)
```

gives

datetime64[ns]
datetime64[ns]
datetime64[ns]
object

While one would expect that the last two lines are identical.
