Quoted keys are not un-quoted

Given the input file...

```toml
[foo]
  "bar" = "baz"
```

... The key `bar` is returned as `"bar"`. To make this a bit clearer:

```ruby
require "toml"
pp TOML.load(<<'__eos__')
[foo]
  "bar" = "baz"
__eos__
# {"foo"=>{"\"bar\""=>"baz"}}
```

The returned hash includes the key `bar` erroneously still wrapped in quotes.