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.

example/advanced/server-flex/advanced_server_flex.cpp

@@ -218,6 +218,26 @@ handle_request(
 void
 fail(beast::error_code ec, char const* what)
 {
+    // ssl::error::stream_truncated, also known as an SSL "short read",
+    // indicates the peer closed the connection without performing the
+    // required closing handshake (for example, Google does this to
+    // improve performance). Generally this can be a security issue,
+    // but if your communication protocol is self-terminated (as
+    // it is with both HTTP and WebSocket) then you may simply
+    // ignore the lack of close_notify.
+    //
+    // https://github.com/boostorg/beast/issues/38
+    //
+    // https://security.stackexchange.com/questions/91435/how-to-handle-a-malicious-ssl-tls-shutdown
+    //
+    // When a short read would cut off the end of an HTTP message,
+    // Beast returns the error beast::http::error::partial_message.
+    // Therefore, if we see a short read here, it has occurred
+    // after the message has been completed, so it is safe to ignore it.
+
+    if(ec == net::ssl::error::stream_truncated)
+        return;
+
     std::cerr << what << ": " << ec.message() << "\n";
 }