Fix key type issue in graphite historian.

There is a wrongful assumption that all the data will be formatted the same way.

Most of the times it looks like this 

```
{.... :tags=>{:ip=>"172.30.5.74", ...}}
```

But sometimes we get stuff like this

```
{.... :tags=>{"ip"=>"172.30.5.74", ...}}
```

This causes us to throw exception, discard the event and produce big log file that fills up the disk( https://github.com/cloudfoundry/cf-release/pull/569 )