servant-client requires charset in Accept PlainText

The current implementation is

```haskell
instance Accept PlainText where
contentType _ = "text" M.// "plain" M./: ("charset", "utf-8")
```

Trying to parse `text/plain` yields `UnsupportedContentType text/plain`

One API I'm calling does not specify the charset so I can't use `PlainText`, but this does the job:

```haskell
data PT
instance Accept PT where
contentType _ = "text" M.// "plain"
```

The workaround is simple, but could/should the charset be made optional?