DateTime as first value in table list throws UndefinedError

When structuring a toml file as:

```
[[entries]]
in = 2019-10-16T13:30:00Z
out = 2019-10-16T17:00:00Z
tags = [
"Elixir"
]
```

The parser fails with: `{:error,
"%Protocol.UndefinedError{description: \"\", protocol: Enumerable, value: ~U[2019-10-16 13:30:00Z]}"}`

When I change the order in the table list as such:

```
[[entries]]
tags = [
"Elixir"
]
in = 2019-10-16T13:30:00Z
out = 2019-10-16T17:00:00Z
```

The parser is happy. 🙂
Any pointers where I could start looking? (I would like to provide a patch for this)