FTS cannot unmarshal query with fields

When creating a FTS query requesting non-string fields (e.g. "*" on a document with many field types), a Go unmarshal error is produced:

cannot unmarshal array into Go struct field SearchResultHit.fields of type string

This is because Fields in the searchResponse struct is set to map[string]string, it should be map[string]interface{} to allow for multiple field types.