UnmarshalKey normalizes all map keys to lower case

```
{
   "someKey": 1,
   "myMap": {
       "A": "v1",
       "B": "v2"
   }
}
```

```
config := viper.New()
config.AddConfigPath("myconfigpath")
config.SetConfigName("myconfig")
```

```
var myMap map[string]string
config.UnmarshalKey("myMap", &myMap)
```

`myMap` has all lower-cased keys. 

Can somebody comment if it is expected or is it a bug ?