Upstreaming RawText from servant-http2-client

@lucasdicioccio Would you be okay with the upstreaming of your `RawText` into `servant`? I am specifically thinking about [this code](https://github.com/lucasdicioccio/servant-http2-client/blob/master/test/Spec.hs#L24-L30):

```haskell
data RawText

instance Accept RawText where
contentType _ = "text/plain"

instance MimeRender RawText Text where
mimeRender _ = fromStrict . Encoding.encodeUtf8

instance MimeUnrender RawText Text where
mimeUnrender _ = pure . Encoding.decodeUtf8 . toStrict
```