pd.concat reset the tz-aware index to UTC
Hi, 

Having two DataFrames with index being tz-aware Timestamp objects 

```
print(concat_df[0].index.tz, concat_df[1].index.tz)
fxt = pd.concat(concat_df, axis=1)
print(fxt.index.tz)

```
This leads to the following output
```
Australia/Melbourne Australia/Melbourne
UTC
```
I believe this may be in some way related to https://github.com/pandas-dev/pandas/issues/7562

pandas version = 0.21.0