Coerce symbol header keys into strings

In my codebase there was a request with an `:Accept` header (as a symbol), and I received an error:
```
TypeError: no implicit conversion of Symbol into String
```

After digging, I found this line. I assume `String.new(k)` was supposed to convert anything stringish into a string. However, `String.new` doesn't really have any abilities, as it does implicit conversion, so it errors even when you pass it a symbol (which is as stringish as you can get :grinning:). So I replaced it with `#String`, which does explicit conversion.