Empty HashMap String fails deserialization but works with serialization

So if I have a `HashMap<String, String>`:
```rs
let mut map: HashMap<String, String>  = HashMap::new();
map.insert("".to_string(), "".to_string());

let str_rep = toml::to_string_pretty(&map).unwrap(); // successful
let map_rep: HashMap<String, String> = toml::from_rep(&str_rep).unwrap(); // fails
```

The toml it creates is:

```toml
"" = ""
```
