This allows the following to be unquoted:
```
foo: {{ bar }}
```

The above is valid YAML but semantically means something like this:
```
{ "foo": { { "bar": null}: null} }
```

This patch looks for events of that form coming from the parser, and
reconstructs the key `bar` into a string node of value "{{ bar }}".

This works properly now:
```
unquoted: {{ foo }}
quoted: {{ 'foo' }}
```

The unquoted jinja feature only works when the unquoted value is valid
YAML.

Don't allow `foo: {{ 'bar' }}` when the internal value is a quoted
string. This form is dubious and probably never seen in the wild. We
just don't give it special treatment so it fails same as before.

Also added check to assert jinja key is a string.