Better treatment of SSL short reads:

fix #38

This improves the behavior when encountering a short read:

* Any stream error encountered during a read is converting into
  `http::error::partial_message` if some data was received but
  the message is incomplete.

* Examples squelch SSL short read errors from the logs.

include/boost/beast/http/impl/write.hpp

@@ -105,6 +105,7 @@ public:
             // What else could it be?
             BOOST_ASSERT(sr_.is_done());
         }
+
         return net::post(
             s_.get_executor(),
             beast::bind_front_handler(