Allowing body to be either string or stream

[ska2342]

When this middleware is used together with other middlewares
it may run into a conflict trying to slurp the body.
If the other middleware already slurped the body and put the
resulting string back into the request the slurp in this middleware
fails.
This commit adds handling of string-type body.

[weavejester]

The request body can only be an InputStream, otherwise it's not a valid Ring request.

Ideally other middleware shouldn't consume the body if they don't need to. Checking the content-type of the request is a good way to avoid consuming request bodies that you don't need to. If you absolutely cannot avoid consuming the body (even after checking the content type!) then replace the body with a ByteArrayInputStream.

[ska2342]

Oh, I wasn't aware that the body has to be an InputStream, probably I was so used to working with the Clojure hash-map, that the JSON middleware gives me.

I must consume all bytes of the body to validate it like this: https://developer.github.com/webhooks/securing/. Then I replaced the body like this:

(assoc request :body (io/input-stream (.getBytes body-string)))

but thought that'd be rather hackish. It seems that this is exactly what you suggest, so I am probably fine with it.
I think we can close this PR then.. Thanks.