Support chunked servlet requests without Content-Length header

HTTP requests with a Transfer-Encoding: chunked body have no Content-Length header. This causes spray 1.0-M7 to discard the body completely.

This patch will read up to settings.MaxContentLength bytes directly from the input stream.

It also updates the debug toString() of HttpEntity to properly abbreviate, and not output any binary data.

Original work by me, OK to release using the Apache 2 license,