validateEveryToJSON wants ToJSON even for ReqBody

With the following API
```hs
type UserAPI
= (Capture "user_id" UserId :> Get '[JSON] User)
:<|> (Capture "new" :> ReqBody '[JSON] NewUser :> Post '[JSON] UserId)
```
and only the following instances
```
newtype UserId = ...
instance FromHttpApiData UserId
instance ToJSON UserId

data User = ...
instance ToJSON User

data NewUser = ...
instance FromJSON NewUser
```

It is not possible to use `validateEveryToJSON` because it expects `instance ToJSON NewUser` even though the `UserAPI` is perfectly usable as `NewUser` is only used in `ReqBody` and thus only requires a `FromJSON` instance.

Of course an easy fix is to just add a `ToJSON` instance, which is low risk compared to adding `FromJSON` instances for things you only want to send out.

Start by locating validateEveryToJSON and reproducing the UserAPI shown in the issue. Verify that a ReqBody with only FromJSON is accepted without requiring ToJSON, while response types still retain their serialization requirements.

Written by the indexing model from the issue text.