Add a test for streaming of response bodies using lazy Bytestrings in servant-server

Currently unfortunately this is not supported. MimeRender is the class that is used to create response bodies. mimeRender returns lazy ByteStrings so servant-server could stream the response lazily, but I have no idea whether it does that or not. And even if it did we have no test making sure that this property will not break at some point.

Btw, there's a test that makes sure streaming of request bodies works in servant-server: StreamingSpec.hs. I think a test for streaming responses would look similar.