Add tests for multiple access of request body

[imalsogreg]

Snap request bodies are InputStream ByteString (or similar) io-streams, and can't be consumed more than once (same issue as for the warp servant server: haskell-servant/servant#3). This can be fixed by, when a request body is consumed, building a new Request from the old and seeding it with a new io-stream build from the consumed ByteString.

This shouldn't be hard, but we will have to force a size limit on the request body (which we want to be configurable when the server starts, through the normal snap config machinery). We will need to use the request-building functions from an Internal module of snap-core, because this kind of request-monkeying is usually something snap users are supposed to avoid doing.

[imalsogreg]

This should be fixed since I've copied over Delayed routing; but leaving this issue open until I write a test to verify.