Fix UnmarshalKey to use of defaultDecoderConfig

viper.go

@@ -713,7 +713,15 @@ func (v *Viper) GetSizeInBytes(key string) uint {
 // UnmarshalKey takes a single key and unmarshals it into a Struct.
 func UnmarshalKey(key string, rawVal interface{}) error { return v.UnmarshalKey(key, rawVal) }
 func (v *Viper) UnmarshalKey(key string, rawVal interface{}) error {
-	return mapstructure.Decode(v.Get(key), rawVal)
+	err := decode(v.Get(key), defaultDecoderConfig(rawVal))
+
+	if err != nil {
+		return err
+	}
+
+	v.insensitiviseMaps()
+
+	return nil
 }
 
 // Unmarshal unmarshals the config into a Struct. Make sure that the tags