Fix BodyTypes to work with generalized ReqBody'

[fizruk]

I've recently discovered that validateEveryToJSON ignored my ReqBody '[Description "...", ...] types. This was because it only considered ReqBody which recently became a synonym for ReqBody '[Required, Strict] in servant-0.13. This PR fixes that.

If you can't update to the latest version of servant-swagger, you can use this workaround locally in the test suite:

-- | Replace every @'ReqBody'' mods@ with 'ReqBody'
-- so that 'validateEveryToJSON' will work for request body types.
type family PatchAPI api where
  PatchAPI (ReqBody' mods cs a :> api) = ReqBody cs a :> PatchAPI api
  PatchAPI ((param :: k) :> api) = param :> PatchAPI api
  PatchAPI (left :<|> right) = PatchAPI left :<|> PatchAPI right
  PatchAPI api = api

...

spec :: Spec
spec = do
  describe "Swagger spec for MyAPI" $ do
    context "ToJSON matches ToSchema" $ do
      validateEveryToJSON (Proxy :: Proxy (PatchAPI MyAPI))
  ...

[fizruk]

It would nice if there was a warning/type error for this sort of breakages, but I'm not sure how to implement one yet.

[phadej]

Something implies TypeSynonymInstances, I have wondered that (not too hard though) before. Maybe it'sFlexibleInstances?

[fizruk]

@phadej Yes, according to GHC documentation:

Extension	Description
TypeSynonymInstances	Enable type synonyms in instance heads. Implied by FlexibleInstances.

However, we don't have either in this module, so I assume it is not needed for type family instances.