Rename identical column names

At present InfluxDB doesn't support column aliases. So, the following query:

~~~
select count(distinct(user_id)), count(user_id) from events
~~~

return dataset with the following column names: ["count", "count"]. It is cause problem because `denormalize_series` function convert dataset to hash with one `count` field.

To avoid this problem I have made a patch which rename identical columns.

Could you please verify this pull request and approve it?