`writes` produces incorrect hcl: omits quotes around in map keys containing `/`

Using version 7.2.1 

The following Python code:

```python
import hcl2

my_hcl = """toto = {
    "john/doh" = {}
} 
"""

tf_vars = hcl2.loads(my_hcl)
hcl2.writes(hcl2.reverse_transform(tf_vars))
```

Produces:
```
'toto = {\n  john/doh = {}\n}\n'
```

hence not keeping the quotes on`"john/doh"` and generating incorrect hcl.