Call handler without waiting for entire body

We were facing issue #90 where our server would wait for the entire body to be received before calling the handler. Since the body is passed to the handler as an `InputStream`, it makes more sense to have the handler being called right after the headers are decoded.

Here is a fix along with a new test case. We had to modify the handler for `test-chunked-encoding`, because in the case of a chunked request, the content-length is still unknown by the time the handler is called.