Support providing a `Content-Length` to `ContentSource`

[parsonsmatt]

We are doing some pass-through requests in our application, where we make a request to S3 and then stream the response back to the client. We want to provide Content-Length headers, since S3 gives us those.

ContentSource only has the ConduitT, and does not provide a way to set the length, even when we know it.

We can provide this with a backwards-compatible pattern synonym and new constructor on Content.

data Content
    = ContentSourceWithLength !(Conduit () (Flush BB.Builder) (ResourceT IO) ()) !(Maybe Int)
    | etc

pattern ContentSource c <- ContentSourceWithLength c _ where
  ContentSource c = ContentSourceWithLength c Nothing

[stephenjudkins]

It would be good to be able to add any arbitrary headers to a ContentSource if possible, as well!

[parsonsmatt]

Oh you can already do that: addHeader