Fix isUnquotedKey, fixes #3479 - #3480

Fixes labels not being quoted in error messages in some cases where they
should have been. For example:

Input code:
```
test :: { "Oops" :: Int }
test = { }
```

Before:
```
  Type of expression lacks required label Oops.

while checking that expression {}
  has type { Oops :: Int
           }
in value declaration test
```

After:
```
  Type of expression lacks required label "Oops".

while checking that expression {}
  has type { "Oops" :: Int
           }
in value declaration test
```

At the time of writing, there is just this one use of `isUnquotedKey`
throughout the entire compiler, so the refactoring I've done here is
safe.